A method for controlling homogenization of a large cross-section continuous casting billet

Abstract

This invention belongs to the field of continuous casting technology, specifically relating to a method for controlling the homogenization of large-section continuous casting billets. This invention employs a three-stage electromagnetic stirring process: submersible nozzle electromagnetic stirring, crystallizer electromagnetic stirring, and end-stage electromagnetic stirring. The direction of the crystallizer electromagnetic stirring is opposite to that of the submersible nozzle electromagnetic stirring. Combined with a reasonable casting speed and continuous casting cooling process, the solidification end position of the billet is controlled at 500-800mm after the end-stage electromagnetic stirring. This ensures that the liquid core diameter is ≥150mm when the billet passes through the end-stage electromagnetic stirring, effectively guaranteeing the uniformity of the microstructure and composition across the cross-section of the large-section billet, thereby achieving stable and consistent performance of the finished parts.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting technology, specifically relating to a method for controlling the homogenization of large-section continuous casting billets. Background Technology

[0002] During the cooling and solidification of ingots and continuously cast billets, liquid phase flow and solid phase movement occur in the internal liquid and mushy regions. This promotes the formation of macrosegregation inside the ingots and continuously cast billets. Due to differences in material size and cooling methods, the distribution type of macrosegregation also varies.

[0003] Large-section continuously cast billets, due to their large cross-sectional dimensions, high carbon content, and wide solid-liquid two-phase region, require a longer solidification time. During solidification, they are prone to internal defects such as center segregation, shrinkage porosity, and other defects. Severe internal defects significantly impact the product quality of the cast billet, thus limiting further improvements in the quality of high-quality special steel. Macroscopic segregation can lead to uneven transformation of the internal structure of the billet during subsequent heat treatment, easily resulting in brittle cracks in the next process, rendering it scrap. High-quality special steel has strict requirements for macroscopic segregation; therefore, strict control of macroscopic segregation at the source of continuous casting billet production is crucial.

[0004] Due to significant differences in the cooling process between continuously cast billets and ingot casting, the manifestations of macroscopic segregation within the billets also differ. The most representative type of segregation in continuously cast billets is center segregation. Center segregation occurs when, during solidification, selective crystallization causes a redistribution of solute elements between the solid and liquid phases. This results in a non-uniform distribution of solute elements such as C, S, and P across the cross-section and longitudinal section of the billet, with the solute content in the central region being significantly higher than in other areas. It also frequently leads to porosity and shrinkage cavities. Center segregation typically manifests as V-shaped, U-shaped, or dot-line segregation along the casting speed direction on the longitudinal section of the continuously cast billet. Slight center segregation is normal, but as segregation worsens, it can have a significant adverse impact on the quality of the steel.

[0005] Compositional uniformity is a crucial factor affecting the hardenability bandwidth and dispersion of gears, thus influencing product machining and meshing accuracy. Magnetically stirred crystallizers (M-EMS) are an important method to improve the compositional uniformity of gear steel billets. However, previous research and production practice have shown that M-EMS easily forms a negative segregation band within a 5–15 mm area beneath the billet surface, and this segregation increases with increasing M-EMS stirring intensity. It also exacerbates the positive segregation in the central region of the billet to some extent, which is detrimental to improving the internal quality of subsequent products. Summary of the Invention

[0006] The purpose of this invention is to solve the above-mentioned problems and provide a method for homogenization control of large-section continuous casting billets. This invention adopts a three-stage electromagnetic stirring process of immersion nozzle electromagnetic stirring, crystallizer electromagnetic stirring and end electromagnetic stirring, combined with reasonable casting speed and continuous casting cooling process, which effectively ensures the uniformity of microstructure and composition on the cross section of large-section casting billets, thereby achieving stable and consistent performance of finished parts.

[0007] The technical problem to be solved by this invention is achieved by the following technical solution: a method for controlling the homogenization of large-section continuous casting billets, comprising the following steps:

[0008] S1. The immersion-type long nozzle is a straight cylindrical nozzle, and an immersion-type long nozzle electromagnetic stirrer is installed between the tundish and the crystallizer.

[0009] A three-stage electromagnetic stirring process is adopted, consisting of submerged long nozzle electromagnetic stirring, crystallizer electromagnetic stirring, and solidification end electromagnetic stirring. The direction of the submerged long nozzle electromagnetic stirring is opposite to that of the crystallizer electromagnetic stirring. This opposite direction is to counteract the steel rotation caused by the crystallizer electromagnetic stirring. Simple long nozzle electromagnetic stirring or crystallizer electromagnetic stirring alone will cause the composition of the billet to be supercooled at the solidification front, resulting in negative segregation under the billet surface. It will also cause the steel to flow out of proportion, resulting in uneven composition of the billet. Since the submerged long nozzle is a straight cylindrical nozzle, without stirring, the impact depth of the steel will increase, which is not conducive to the uniform control of the composition and temperature of the steel along the length of the billet. At the same time, the temperature distribution of the steel in the crystallizer is prone to be uneven, resulting in uneven melting of the protective slag on the surface of the steel in the crystallizer, which is not conducive to the control of the surface quality of the billet.

[0010] The electromagnetic stirring parameters are set according to the composition of the molten steel. The electromagnetic stirring current of the immersion long nozzle is set to 450-550A and the frequency is 40-60Hz.

[0011] The current for the crystallizer and the end electromagnetic stirrer is determined based on the fluidity of molten steel during the solidification process. The frequency of the crystallizer electromagnetic stirrer is 1-3Hz, and the frequency of the end electromagnetic stirrer is 5-8Hz. Electromagnetic stirring uses electromagnetic force to stir the molten steel and make it rotate. The greater the current, the stronger the stirring force, and the lower the frequency, the deeper the penetration. Depending on the fluidity of the molten steel, the electromagnetic force required to achieve the same stirring effect is different, that is, the current required is different. The frequency required for billets with different cross-sectional diameters is also different.

[0012] In the crystallizer section, the billet shell is relatively thin, so the electromagnetic force does not need to penetrate the thick billet shell, and a relatively low frequency can achieve a good stirring effect. At the end electromagnetic stirring position, most of the billet has solidified, and only the liquid core in the core has not solidified. The electromagnetic force needs to penetrate the thicker billet shell, so a higher electromagnetic stirring frequency is required.

[0013] S2. The superheat of molten steel in the tundish should be controlled within the range of 10-30℃. If the superheat of molten steel is <10℃, there is a greater risk of freezing. If the superheat is >30℃, it is considered high superheat control, which is not conducive to the solidification of molten steel.

[0014] S3. Using a two-level model control, select reasonable cooling parameters and casting speed to control the solidification end position of the billet at 500-800mm after the end electromagnetic stirring, ensuring that the liquid core diameter of the billet is ≥150mm when passing through the end electromagnetic stirring of the solidification.

[0015] S4. The second cooling section adopts air mist cooling and weak cooling water distribution. Air mist cooling and weak cooling water distribution can make the billet cooling more uniform and avoid excessive local cooling, which can lead to stress concentration and uneven structure of the billet.

[0016] This invention employs a three-stage electromagnetic stirring process. By using an immersion nozzle for electromagnetic stirring, molten steel enters the crystallizer in a rotating manner after passing through the immersion nozzle. This improves the temperature uniformity of the molten steel across the cross-section inside the crystallizer and reduces the impact of the molten steel on the lower part of the billet, thereby reducing slag entrapment and mitigating the tendency of slag entrapment during continuous casting. In addition, the direction of the electric stirring in the crystallizer is opposite to that in the immersion nozzle, which changes the flow direction of the molten steel, further homogenizing the composition and temperature of the molten steel. At the same time, it is beneficial to break up dendrites and increase the number of nucleation points inside the billet.

[0017] This invention employs a two-level model to predict the solidification end position of the billet, optimizing the billet's cooling parameters and casting speed. The solidification end position is controlled at 500-800mm after the final electromagnetic stirring, ensuring a liquid core diameter ≥150mm when the billet passes through the final electromagnetic stirring point. This maximizes the effectiveness of the final electromagnetic stirring, promoting uniform composition and temperature in the billet's core and effectively improving defects such as center segregation and shrinkage cavities. Installing the electromagnetic stirrer 500-800mm before the solidification end ensures a specific liquid core diameter, guaranteeing effective stirring. At the solidification end, the molten steel begins to solidify completely, requiring a very large stirring force to agitate the liquid core, which is often difficult to achieve. The ultimate goal is to uniformly distribute the composition and temperature of the molten steel at the solidification end, thereby improving segregation.

[0018] The technical solution of the present invention also includes: in step S1, the specific current scheme for the crystallizer and the end electromagnetic stirrer is as follows:

[0019]

[0020]

[0021]

[0022] All current units mentioned above are in amperes (A). The five elements C, Mn, Cr, Mo, and Ni are used to limit the current because these five elements significantly affect the fluidity of molten steel. These elements lower the liquidus temperature of molten steel to varying degrees, thereby increasing its fluidity. This empirical formula was derived through long-term production experience. However, this formula only applies to medium and low alloy steels. For high alloy steels, a large amount of alloy will form carbides, reducing the fluidity of the molten steel, rendering the formula inapplicable. It should also be noted that the electromagnetic stirring parameters are closely related to the continuous casting machine.

[0023] The technical solution of the present invention also includes: In step S3, the specific scheme for controlling the pulling speed is as follows:

[0024] The casting speed of the cross-section billet is controlled at 0.38-0.48 m / min; The casting speed of the cross-section billet is controlled at 0.20-0.28 m / min; The casting speed of the cross-section billet is controlled at 0.17-0.21 m / min. The casting speed is related to the design of the continuous casting machine and the characteristics of the steel grade. This section focuses on the fixed casting speed control for a specific continuous casting machine.

[0025] The technical solution of the present invention also includes: In step S4, the specific solution for the weak cooling water distribution is as follows:

[0026] The specific water content for low-carbon steel is controlled at 0.15-0.25 L / kg; for medium-carbon steel at 0.08-0.18 L / kg; and for high-carbon steel at 0.05-0.15 L / kg. Steels with different carbon contents have different heat capacities and release different amounts of heat during solidification. Therefore, it is necessary to control the specific water content to ensure a fixed liquid core length for the cast billet.

[0027] The technical solution of the present invention also includes: in step S1, the immersion-type long water inlet electromagnetic stirrer is installed on the outer wall of the immersion-type long water inlet.

[0028] The technical solution of the present invention also includes: the carbon difference of the cross section of the billet obtained by the control method is: ≤0.02% for low carbon steel, ≤0.05% for medium carbon steel, and ≤0.07% for high carbon steel.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] 1. Since the submerged long nozzle is a straight cylindrical nozzle, the molten steel has a certain radial velocity when it flows out of the nozzle. The application of the submerged long nozzle electric stirring process reduces the impact depth of the molten steel, improves the flow field in the crystallizer, increases the temperature of the molten steel at the meniscus, which is conducive to the melting of the protective slag and improves the surface quality of the billet.

[0031] 2. The combined application of immersion-type long nozzle electric agitator and crystallizer electromagnetic agitator, due to the opposite stirring directions, cancels out the effects of the two. After the molten steel passes through the electromagnetic agitator of the crystallizer, it gradually stops rotating, reducing the supercooling factor of the molten steel during the solidification process, improving the microstructure and composition uniformity of the billet, and the carbon difference of the billet cross section: low carbon steel ≤0.02%, medium carbon steel ≤0.05%, high carbon steel ≤0.07%.

[0032] 3. This invention, through a reasonable continuous casting process, by setting reasonable end electromagnetic stirring parameters and reasonably controlling the end electromagnetic stirring position, is conducive to giving full play to the role of end electromagnetic stirring, thereby improving the low magnification quality of the billet, with the central porosity of the billet ≤1.0 grade and the shrinkage cavity ≤0.5 grade. Attached Figure Description

[0033] Figure 1 This is a low-magnification metallographic image of the cast billet in Embodiment 1 of the present invention;

[0034] Figure 2 This is a low-magnification metallographic image of the cast billet in Embodiment 2 of the present invention;

[0035] Figure 3 This is a low-magnification metallographic image of the cast billet in Embodiment 3 of the present invention;

[0036] Figure 4 This is a low-magnification metallographic image of the cast billet in Comparative Example 1 of this invention;

[0037] Figure 5 This is a schematic diagram of the sampling of the carbon range of the low-magnification cross-section of the billet in this invention. Detailed Implementation

[0038] The present invention will be further described below with reference to embodiments.

[0039] Example 1

[0040] The large-section continuous casting billet homogenization control method of the present invention is used to cast a cross-section with a diameter of [missing information]. The specific production process parameters for 9Cr5Mo steel are shown in Table 1, and the low-magnification rating results of the cast billets are shown in Table 2. Figure 1 As shown.

[0041] Example 2

[0042] The large-section continuous casting billet homogenization control method of the present invention is used to cast a cross-section with a diameter of [missing information]. The specific production process parameters for 42CrMoA steel are shown in Table 1, and the low-magnification rating results of the cast billets are shown in Table 2 and... Figure 2 As shown.

[0043] Example 3

[0044] The large-section continuous casting billet homogenization control method of the present invention is used to cast a cross-section with a diameter of [missing information]. The specific production process parameters for 18CrNiMo7-6 steel are shown in Table 1, and the low-magnification rating results of the cast billets are shown in Table 2. Figure 3 As shown.

[0045] Comparative Example 1

[0046] Using existing technology, the diameter of the casting section is The specific production process parameters for 42CrMoA steel are shown in Table 1, and the low-magnification rating results of the cast billets are shown in Table 2 and... Figure 4 As shown.

[0047] In this invention, the sampling method for the carbon range of the low-magnification cross-section of the cast billet is described in [reference needed]. Figure 5 .

[0048] Table 1 Process Parameters

[0049]

[0050] Table 2 Low-rating results

[0051]

[0052] Based on Tables 1 and 2, it can be concluded that Example 2 and Comparative Example 1 produce the same steel grade. However, since the present invention adopts a three-stage electromagnetic stirring process of immersion long nozzle electromagnetic stirring, crystallizer electromagnetic stirring and solidification end electromagnetic stirring, and the direction of immersion long nozzle electromagnetic stirring is opposite to that of crystallizer electromagnetic stirring, the carbon difference of the low magnification cross section of the billet is significantly reduced in Example 2 compared with Comparative Example 1.

[0053] Furthermore, Examples 1-3 set different process parameters according to different steel compositions and steel diameters, which satisfied the homogenization control of large cross-section continuous casting billets of different diameters.

Claims

1. A method for controlling the homogenization of large-section continuously cast billets, characterized in that, Includes the following steps: S1. The immersion-type long nozzle is a straight cylindrical nozzle, and an immersion-type long nozzle electromagnetic stirrer is installed between the tundish and the crystallizer. A three-stage electromagnetic stirring process is adopted, consisting of immersion-type long nozzle electromagnetic stirring, crystallizer electromagnetic stirring, and solidification end electromagnetic stirring. The direction of immersion-type long nozzle electromagnetic stirring is opposite to that of crystallizer electromagnetic stirring. The electromagnetic stirring parameters are set according to the composition of the molten steel. The electromagnetic stirring current of the immersion long nozzle is set to 450-550A and the frequency is 40-60Hz. The current of the crystallizer and the end electromagnetic stirrer is determined based on the fluidity of molten steel during the solidification process. The frequency of the crystallizer electric stirring is 1-3Hz, and the frequency of the end electromagnetic stirrer is 5-8Hz. S2. The superheat of the molten steel in the tundish should be controlled within the range of 10-30℃; S3. Using a two-level model control, select reasonable cooling parameters and casting speed to control the solidification end position of the billet at 500-800mm after the end electromagnetic stirring, ensuring that the liquid core diameter of the billet is ≥150mm when passing through the end electromagnetic stirring of the solidification. S4, the second cooling section uses air mist cooling and weak cooling water distribution; In step S1, the specific current scheme for the crystallizer and the end electromagnetic stirrer is as follows: The current of the ≤φ500mm cross-section billet crystallizer and the end electromagnetic stirrer is ≤ ; The current of the φ650mm cross-section billet crystallizer and the end electromagnetic stirrer is ≤ ; The current of the ≤φ800mm cross-section billet crystallizer and the end electromagnetic stirrer is ≤ ; In step S3, the specific scheme for speed control is as follows: The casting speed for φ500 cross-section billets is controlled at 0.38-0.48 m / min; the casting speed for φ650 cross-section billets is controlled at 0.20-0.28 m / min; and the casting speed for φ800 cross-section billets is controlled at 0.17-0.21 m / min. In step S4, the specific scheme for weak cold water distribution is as follows: The water content for low-carbon steel should be controlled at 0.15-0.25 L / kg; for medium-carbon steel at 0.08-0.18 L / kg; and for high-carbon steel at 0.05-0.15 L / kg.

2. The method for controlling the homogenization of large-section continuously cast billets according to claim 1, characterized in that: In step S1, the immersion-type long nozzle electromagnetic stirrer is installed on the outer wall of the immersion-type long nozzle.

3. The method for controlling the homogenization of large-section continuously cast billets according to claim 1, characterized in that: The carbon tolerance of the cross-section of the billet obtained by the control method is as follows: ≤0.02% for low carbon steel, ≤0.05% for medium carbon steel, and ≤0.07% for high carbon steel.

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