A high-homogenization medium- and low-carbon alloy steel large-section rectangular billet and its preparation method

Abstract

The present invention provides a highly homogenized medium- and low-carbon alloy steel large-section rectangular billet and preparation method, belonging to the field of special steel continuous casting. The medium- and low-carbon alloy steel has a carbon content of 0.2-0.45% by mass, alloy Cr and Mn contents of 0.5-1.1% by mass, and a Ti content of 0.01-0.1% by mass, with a cross-section of 320-390 mm x 385-510 mm. The steel adopts an electromagnetic stirring mode that matches weak crystallizer electromagnetic stirring with strong solidification end electromagnetic stirring at a stirring frequency determined by the maximum electromagnetic stirring force, combined with a relatively strong secondary cooling water ratio, and a position of the electromagnetic stirrer at the end of solidification at a center solid phase ratio of 0.1-0.2, a light reduction of 0.25-0.85 and a reduction of 5-12 mm, and a heavy reduction of 0.9-1© and a reduction of 5-6 mm. The present invention can significantly improve the compositional uniformity of the ingot cross section and improve low-magnification frame segregation.

Description

Technical Field

[0001] The invention belongs to the field of special steel continuous casting, and in particular relates to a highly homogenized medium- and low-carbon alloy steel large-section rectangular billet and a preparation method thereof. Background Art

[0002] Medium- and low-carbon alloy steels have a carbon content of 0.2% to 0.45% and contain alloying elements such as Mn, Cr, Mo, and Ti. The solidification characteristics of large-section rectangular billets produced during continuous casting are characterized by coarse and well-developed columnar crystals, a large columnar-to-equiaxed transition zone, and a low distribution coefficient of solute carbon between the solid and liquid phases. Furthermore, during solidification, the formation and growth of columnar, equiaxed, and mixed crystal zones differ in their orientation, both along the inner and outer arcs and in the left and right directions of the cross-section. Carbon segregation also differs between the wide and narrow sides of the billet. This is manifested as a black frame on a low-magnification cross-section image of the billet, indicating frame-type segregation. This occurs at a position 1 / 2 to 3 / 4 from the center of the billet (the columnar-to-equiaxed transition zone (CET)), where the highest carbon content is observed. Frame-type segregation is more severe in large-section rectangular billets than in small square billets, and becomes more pronounced with larger cross-sections. Currently, high-quality steel companies' large-section rectangular billet continuous casting machines are typically equipped with mold electromagnetic stirring, end-of-solidification electromagnetic stirring, and light and heavy reduction technologies. Mismatching the casting speed, secondary cooling process, mold and end-of-solidification electromagnetic stirring parameters, and light and heavy reduction process parameters can lead to frame-shaped segregation, resulting in large carbon extremes and poor uniformity across the cross-section of the billet. This leads to minimal improvement during the subsequent heating and rolling processes, resulting in ingot-shaped segregation in the large bar. This can lead to poor hardenability, quenching deformation, and cracking during subsequent heat treatment.

[0003] Extensive research has been conducted to improve the frame segregation problem in large-section rectangular billets of medium- and low-carbon alloy steel. In continuous casting, these efforts primarily involve reducing electromagnetic stirring in the mold and increasing the secondary cooling water ratio to reduce central equiaxed crystals and increase columnar crystals, promoting the propagation of CET into the ingot. Furthermore, adjustments are made to the secondary cooling water distribution on the wide and narrow sides to minimize the frame segregation from rectangular to square. Stronger electromagnetic stirring at the end of solidification is used to refine the central equiaxed crystals and stir the liquid core for a more uniform composition, thereby reducing central shrinkage and porosity. This approach, combined with electromagnetic stirring in the mold, has limited success. For example, Chinese patent application number 202311485421.3 discloses a continuous casting method for improving carbon segregation of spring steel rectangular billets. It proposes specific crystallizer cooling water volume, secondary cooling water ratio and distribution ratio of each zone, stirring current and frequency of crystallizer and electromagnetic stirring at the end of solidification, corresponding reduction amount of each straightening machine under light reduction, and temperature requirements of heating and rolling processes. However, there is no definite rule for optimizing the process parameters of electromagnetic stirring of crystallizer and solidification end, whether the casting speed is reasonably matched with the electromagnetic stirring position at the end of solidification and the reduction position under light reduction, and whether the rationality of the electromagnetic stirring position at the end of solidification and the position of several straightening machines performing light reduction is not explained. Different casting machines have different technical devices and process arrangements, resulting in no reference for implementation on other similar casting machines, and poor universality of the scheme. At the same time, its embodiment does not provide a low-magnification of the cross-section of the billet, and it is impossible to determine whether the frame segregation of the billet is improved. Summary of the Invention

[0004] Based on the above problems existing in the prior art, the embodiment of the present invention provides a highly homogenized medium and low carbon alloy steel large cross-section rectangular billet and a preparation method, which improves the problems existing in the existing technology of improving the frame segregation and cross-section carbon extreme difference of medium and low carbon alloy steel large cross-section rectangular billet at a higher drawing speed, including the optimization and determination rules of the crystallizer and the electromagnetic stirring process parameters at the end of solidification, the accuracy verification of the solidification heat transfer model, the rational matching of the drawing speed and the electromagnetic stirring at the end of solidification and the light and heavy pressing positions, and the process technology for the frame segregation and cross-section carbon extreme difference of the rectangular billet. Improvements are made by quantitative evaluation of surface carbon range, etc., to provide a preparation method for highly homogenized large-section rectangular billets of medium and low carbon alloy steel, based on which the solidification heat transfer model and solidification structure model of the billet are established, and the pulling speed, secondary cooling water ratio, electromagnetic stirring between the crystallizer and the solidification end, and the reasonable matching of light reduction and heavy reduction process parameters are coordinated to accurately control the solidification structure of the billet, improve the uniformity of the cross-sectional composition of the large-section rectangular billet of medium and low carbon alloy steel, control the cross-sectional carbon range below 0.03%~0.04%, and significantly reduce the low-magnification frame segregation of the cross section of the billet.

[0005] The highly homogenized medium- and low-carbon alloy steel large-section rectangular billet and its production method in the embodiments of the present invention utilizes a multi-field synergy of thermal, electromagnetic, and mechanical stress fields. The mechanical stress field involves light and heavy reduction operations at the end of solidification. For large-section rectangular billets of medium and low carbon alloy steel, considering the production rhythm and efficiency, medium and low carbon alloy steel generally has a higher pulling speed, the liquid core becomes larger and the solidification end point is prolonged. In addition, compared with high carbon steel, the two-phase zone of medium and low carbon alloy steel is shorter, the effective electromagnetic stirring position range at the end of solidification is narrow, and there are fewer straightening rollers corresponding to the light pressure compressible range, resulting in frame segregation and unstable cross-sectional carbon range control; electromagnetic stirring stirs the molten steel through the electromagnetic force generated by electromagnetic induction with the molten steel. The stirring force is the main parameter to measure the stirring capacity. Due to the shielding effect of the crystallizer copper tube and the solidified billet shell on the magnetic field and the skin effect, the stirring current and stirring frequency cannot directly reflect the stirring capacity; the electromagnetic stirring and light and heavy pressure at the end of solidification must give full play to their role, which is closely related to the installation position of the end electromagnetic stirring and the layout of the light pressure and heavy pressure straightening machine. Its reasonable matching with the pulling speed is a prerequisite.

[0006] For large-section rectangular billets of medium- and low-carbon alloy steel, reducing the intensity of electromagnetic stirring in the crystallizer, increasing the secondary cooling water ratio, and adjusting the secondary cooling water distribution increase the number of columnar crystals and slow their transformation to equiaxed crystals, hindering the carbon from advancing into the billet and diffusing toward the edges. Strong alternating electromagnetic stirring at the end of solidification slowly reduces the carbon content of the molten steel at the larger solidification front in the liquid core, allowing it to diffuse toward the core and refine the solidification structure. Light reduction at the end of solidification with a small reduction compensates for shrinkage, promotes the flow of concentrated molten steel between dendrites, improves center segregation, and reduces center shrinkage and porosity. Applying large reductions near the solidification position under heavy pressure significantly improves center shrinkage and porosity, increasing center density.

[0007] In order to achieve the above objectives, the embodiments of the present invention adopt the following technical solutions:

[0008] In a first aspect, an embodiment of the present invention provides a method for preparing a highly homogenized large-section rectangular billet of medium- and low-carbon alloy steel.

[0009] The preparation method is designed to solve the problems of poor hardenability of the bars and quenching deformation and cracking caused by large cross-section carbon extreme difference and poor uniformity in large-section rectangular billets for existing medium- and low-carbon alloy steel bars. The mass percentages of the elements in the medium- and low-carbon alloy steel are C: 0.2% to 0.45%, Mn: 0.5% to 1.0%, Cr: 0.8% to 1.1%, Mo: 0.05% to 0.2%, and Ti: 0.01% to 0.1%. During the continuous casting process of large square billets, based on the theoretically optimized casting speed, secondary cooling water ratio, and electromagnetic stirring current range, weaker mold electromagnetic stirring (center magnetic induction intensity 100-200 Gs) and stronger secondary cooling water ratio (0.13-0.26 L / Kg) at the optimal stirring frequency are used to reduce the central equiaxed grains and slow the transformation of columnar grains to equiaxed grains, thereby hindering the advancement of carbon into the ingot and allowing it to diffuse toward the edges. Stronger alternating solidification end electromagnetic stirring (center magnetic induction intensity 200-300 Gs) at the optimal stirring frequency slowly reduces the carbon content of the molten steel at the solidification front, which is larger in the liquid core, allowing it to diffuse toward the core and refine the solidification structure. Multiple straightening machines apply light reduction (5-12 mm) with small reductions to promote the flow of solute-enriched molten steel in the liquid core, improving central segregation and compensating for shrinkage, suppressing shrinkage cavity formation. Single-roll heavy reduction reduces shrinkage cavities and looseness in the ingot center, thereby increasing central density. The systematic application of these technical measures ultimately improves frame segregation and cross-sectional composition uniformity. The center solid fraction of the slab, calculated using a validated solidification heat transfer model, determines the final electromagnetic stirring position (center solid fraction 0.1-0.2), the light reduction position (center solid fraction 0.25-0.85), and the heavy reduction position (center solid fraction 0.9-1), and these are appropriately matched with the casting speed. This invention, based on a validated slab solidification heat transfer model and solidification structure model, coordinates the casting speed, secondary cooling water ratio, the crystallizer and the end-solidification electromagnetic stirring, and the light and heavy reduction process parameters to precisely control the distribution of the solidification structure and carbon element composition of the slab. This improves the cross-sectional composition uniformity of large-section rectangular slabs of medium- and low-carbon alloy steel, controls the cross-sectional carbon range to below 0.03%-0.04%, and significantly reduces the frame-shaped segregation of the slab's solidification macrostructure.

[0010] like Figure 1 As shown, the preparation method comprises:

[0011] Step S1: electromagnetic stirrers are respectively provided at the crystallizer and the solidification end of the continuous casting machine, and 9 to 10 straightening machines are provided at the solidification end.

[0012] In this step, 9 to 10 straightening machines have a light reduction function, and the maximum reduction of a single roller of the straightening machine implementing light reduction is not less than 5 mm. Among them, 4 to 5 straightening machines have a heavy reduction function, and the maximum reduction of a single roller of the straightening machine implementing heavy reduction is not less than 10 mm, and the maximum pressure is 2500KN.

[0013] Step S2, based on physical equipment including electromagnetic stirring, couple the solidification heat transfer model with the cellular automaton model to construct and verify the solidification structure model of medium-carbon low-alloy steel in large-section rectangular billets. Taking the preset billet solidification structure as the target, the drawing speed range, secondary cooling water ratio range, and stirring current range of the crystallizer and the electromagnetic stirrer at the end of solidification of the medium-carbon low-alloy steel large-section rectangular billet are optimized.

[0014] In this step, the element contents in the medium- and low-carbon alloy steel are as follows: C: 0.2%-0.45%, Mn: 0.5%-1.0%, Cr: 0.8%-1.1%, Mo: 0.05%-0.2%, and Ti: 0.01%-0.1%. The casting machine is a full-arc 5-machine, 5-strand continuous casting machine, with a slab cross-sectional size of 320-390 mm x 385-510 mm. Through model prediction and verification, the slab's macroscopic solidification structure (columnar crystal ratio, mixed crystal region ratio, and central equiaxed crystal ratio) is accurately predicted.

[0015] The casting speed is precisely determined based on the center solid fraction calculated by the solidification heat transfer model, the required electromagnetic stirring position at the end of solidification (corresponding to a center solid fraction of 0.1-0.2), the light reduction position (corresponding to a center solid fraction of 0.25-0.85), and the heavy reduction position (corresponding to a center solid fraction of 0.9-1). In a preferred embodiment, through model calculation and verification, the optimized continuous casting process parameters are: a casting speed of 0.39-0.6 m / min, a stronger weak cooling mode for secondary cooling, and a water content of 0.13-0.26 L / Kg.

[0016] Step S3, determine the optimal stirring frequency of the crystallizer electromagnetic stirrer; determine the setting position and optimal stirring frequency of the electromagnetic stirrer at the end of solidification; determine the pressing position and single roller pressing amount of the light pressing and heavy pressing of the solidification end tensioning and leveling machine.

[0017] In this step, determining the optimal stirring frequency includes:

[0018] Step S301, using a Gaussmeter with an alternating low-frequency mode to measure the center magnetic induction intensity of the crystallizer electromagnetic stirrer and the solidification end electromagnetic stirrer in an alternating low-frequency mode offline, and a copper tube is required when measuring the crystallizer electromagnetic stirrer. Preferably, the Gaussmeter uses a Lakeshore 475DS Gaussmeter. In a preferred embodiment, the measurement determines the stirring current corresponding to the central magnetic induction intensity of the crystallizer electromagnetic stirrer being 100-200Gs, and the stirring current corresponding to the central magnetic induction intensity of the solidification end stirrer being 200Gs-300Gs.

[0019] Step S302 , calculating the optimal stirring frequencies of the crystallizer electromagnetic stirrer and the solidification end electromagnetic stirrer according to the central magnetic induction intensity.

[0020] Specifically, the measured central magnetic induction intensity is substituted into formula (1):

[0021] (1)

[0022] In formula (1), T is the electromagnetic torque, σ is the conductivity of the conductor, f is the stirring frequency, B is the measured magnetic induction intensity at the center of the stirrer, r1 is the equivalent radius of the liquid core, and L is the effective radius of the stirrer.

[0023] For the crystallizer and the electromagnetic stirrer at the end of solidification, the stirring current is positively correlated with the central magnetic induction intensity B, while the stirring frequency f is negatively correlated with the central magnetic induction intensity B and the stirring frequency f. A predetermined negative correlation exists between the two. The magnetic induction intensity at the center of the stirrer is measured using a Gaussmeter at different stirring currents and frequencies, and then fitted to obtain the relationship between the central magnetic induction intensity B and the frequency f. Therefore, due to the correlation between B and f in Equation (1), the electromagnetic torque T reaches its maximum value relative to the variable f. The frequency corresponding to the maximum electromagnetic torque T is the optimal stirring frequency.

[0024] In this step, the location of the electromagnetic stirrer at the end of solidification is determined based on the solid phase ratio at the center of the ingot. The solid phase ratio at the center of the ingot at the location where the electromagnetic stirrer is installed is 0.1-0.2.

[0025] In this step, when determining the light reduction and heavy reduction positions and single-roll reduction amounts of the drawing and straightening machines at the end of solidification, 2 to 3 drawing and straightening machines perform light reduction, and one drawing and straightening machine performs heavy reduction near the light reduction area. The reduction process determines the reduction position and single-roll reduction amount according to the solid phase ratio at the center of the ingot. The solid phase ratio at the center of the ingot at the light reduction position is 0.25 to 0.85, and the total reduction amount for light reduction is 5 to 12 mm; the solid phase ratio at the center of the ingot at the heavy reduction position is 0.9 to 1, and the single-roll reduction amount of one heavy-pressure drawing and straightening machine is 5 mm to 6 mm, that is, the total reduction amount for heavy reduction is 5 to 6 mm.

[0026] The center solid fraction of the ingot is calculated using a solidification heat transfer model verified by temperature measurement and nail-shooting tests. The relationship between the reduction under light and heavy pressure and the center solid fraction of the ingot is as follows: for light pressure, when the center solid fraction of the ingot is in the range of 0.25-0.5, the single-roll reduction is 1-3 mm; for light pressure, when the center solid fraction is in the range of 0.5-0.85, the single-roll reduction is 3-4 mm, and the total light pressure reduction is 5-12 mm. For heavy pressure, when the center solid fraction of the ingot is in the range of 0.9-1, the single-roll reduction is 5-6 mm, and the total heavy pressure reduction is 5-6 mm.

[0027] Step S4, after the continuous casting machine starts casting, in the stage from the tundish to the crystallizer: the molten medium- and low-carbon alloy steel flows from the tundish of the multi-stream large-section rectangular billet continuous casting machine into the crystallizer through the submerged nozzle;

[0028] Step S5, from the crystallizer to the end of solidification: the crystallizer electromagnetic stirrer rotates and stirs the molten steel, and solidifies it in the secondary cooling zone by mist cooling; at the set position of the solidification end, the solidification end electromagnetic stirrer stirs the molten steel in the core mushy zone;

[0029] Step S6, from the end of solidification to the stage of casting forming: the casting containing the liquid core that is not completely solidified is bent and straightened by multiple straightening machines; according to the set light reduction position and the corresponding single-roll reduction amount, light reduction is performed at the straightening machine corresponding to the light reduction position with the corresponding light reduction single-roll reduction amount; when approaching the end of solidification, according to the set heavy reduction position and the corresponding single-roll reduction amount, heavy reduction is performed at the straightening machine corresponding to the heavy reduction position with the corresponding heavy reduction single-roll reduction amount;

[0030] Step S7, for the obtained medium-low carbon alloy steel large-section rectangular billet, the performance evaluation is performed by using the low-magnification morphology and rating of the billet cross section and longitudinal section, the carbon composition of the cross section drilling, the metal in-situ analysis of the cross section carbon composition distribution and the quantitative central density.

[0031] In a second aspect, an embodiment of the present invention further provides a highly homogenized medium and low carbon alloy steel large cross-section rectangular billet, which is prepared using the method for preparing a highly homogenized medium and low carbon alloy steel large cross-section rectangular billet as described above.

[0032] The solution of the embodiment of the present invention has the following beneficial effects:

[0033] The embodiment of the present invention provides a highly homogenized medium and low carbon alloy steel large cross-section rectangular billet and preparation method. Secondary cooling and electromagnetic stirring of the crystallizer increase columnar crystals and slow down the transformation of columnar crystals to equiaxed crystals; electromagnetic stirring at the end of solidification refines the central equiaxed crystals and improves central segregation; light pressure at the end of solidification improves central segregation of the billet and reduces central shrinkage, and heavy pressure at the end of solidification significantly improves central shrinkage and looseness of the billet; the synergy of thermal stress field, electromagnetic field and mechanical stress field can stably control the frame segregation and cross-section carbon extreme difference of the medium and low carbon alloy steel large cross-section rectangular billet to the greatest extent, which can significantly improve the heat treatment process and use it as raw material for production The problems of poor hardenability of large bars and deformation and cracking during quenching; through the establishment and verification of solidification heat transfer and solidification structure models, the performance differences of different continuous casting machine technical equipment and the differences in casting machine process design layout can be taken into account, and a replicable method for determining continuous casting process parameters (drawing speed, electromagnetic stirring of crystallizer and solidification end, light pressure and heavy pressure) can be formulated, and the effect of controlling the solidification structure of the ingot at a higher drawing speed, improving frame segregation and cross-section carbon extremes, without the need for a large number of comparative tests, improving efficiency, saving time and cost, and the process flow is universal and can be realized in industrial production.

[0034] Of course, it is not necessary to achieve all of the advantages described above simultaneously in order to implement any product or method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0036] Figure 1 This is a flow chart of the preparation method according to an embodiment of the present invention;

[0037] Figure 2 Schematic diagram of low-magnification sampling of the cross section of the ingot prepared in an embodiment of the present invention;

[0038] Figure 3 Schematic diagram of carbon composition sampling of a cross section of a slab prepared in an embodiment of the present invention;

[0039] Figure 4 A solidification structure diagram of a casting blank simulated by the solidification structure model established in an embodiment of the present invention;

[0040] Figure 5 This is an in-situ scanning carbon composition distribution diagram of the cross section of the ingot in Example 1 of the present invention;

[0041] Figure 6 This is the density distribution of the in-situ scanning of the cross section of the ingot in Example 1 of the present invention;

[0042] Figure 7 This is a carbon composition distribution diagram of the inner and outer arc center lines of the ingot in Example 1 of the present invention;

[0043] Figure 8 The carbon composition distribution of the in-situ scanning of the cross-section of the ingot in Example 2 of the present invention is shown;

[0044] Figure 9 The in-situ scanning density distribution of the cross-section metal of the ingot in Example 2 of the present invention is shown;

[0045] Figure 10 The carbon composition distribution of the inner and outer arc center lines of the ingot in Example 2 of the present invention;

[0046] Figure 11 This is the in-situ scanning of carbon composition distribution of the cross-section metal of the ingot in the comparative example of the present invention;

[0047] Figure 12 The in-situ scanning density distribution of the cross-section metal of the ingot in the comparative example of the present invention is shown;

[0048] Figure 13 This is a carbon composition distribution diagram of the inner and outer arc center lines of the ingot in the comparative example of the present invention. DETAILED DESCRIPTION

[0049] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations. It should be noted that the embodiments of the present invention and the features in the embodiments can also be combined with each other in the absence of conflict.

[0050] It should be noted that similar reference numerals and letters represent similar items in the following figures. Therefore, once an item is defined in one figure, it does not need to be further defined or explained in subsequent figures. In the description of the present invention, the terms "first," "second," "third," "fourth," etc. are used only to distinguish the description and are not to be understood as indicating or implying relative importance.

[0051] In Example 1, Example 2 and Comparative Example 1, the main equipment parameters and technical configurations of the continuous casting machine are shown in Table 1.

[0052] Table 1 Main equipment parameters and technical configuration of continuous casting machine

[0053]

[0054] In order to evaluate the macro solidification structure, frame segregation and cross-section carbon extreme of the ingot, the sampling method for the ingot is as follows: Figure 2 and Figure 3 As shown in the figure, the inner side refers to the inner arc of the ingot, and the outer side refers to the outer arc of the ingot. Low-magnification images were obtained using the hot pickling method; carbon composition was measured using the borehole carbon-sulfur analyzer method; and carbon composition distribution and density of the ingot cross section were obtained using the metal in-situ analyzer OPA scanning.

[0055] Example 1

[0056] This embodiment provides a highly homogenized medium-low carbon alloy steel large cross-section rectangular billet and its preparation method. Medium carbon alloy steel 42CrMoA is used, and the specific composition is shown in Table 2:

[0057] Table 2 Main chemical composition of medium carbon alloy steel 42CrMoA (wt%)

[0058]

[0059] The preparation method comprises:

[0060] Equipment determination:

[0061] Electromagnetic stirrers are installed in the crystallizer and at the end of solidification, and 9 straightening machines are set up at the end of solidification. All 9 straightening machines have light reduction function. The maximum reduction of a single roller of the straightening machine is not less than 5mm. Among them, 4 straightening machines have heavy reduction function. The heavy reduction straightening machines are flat rollers. The maximum reduction of a single roller of the straightening machine is not less than 10mm and the maximum pressure is 2500KN.

[0062] Determination of steel grade process parameters:

[0063] A solidification structure model for medium-carbon, low-alloy steel in a large rectangular billet, coupled with electromagnetic stirring and heat transfer, was developed and validated using cellular automation. The casting speed range, secondary cooling water ratio, and electromagnetic stirring current range for the large rectangular billet were optimized, targeting the desired solidification structure. In this example, the medium-carbon alloy steel, 42CrMoA, was cast using a BOF-to-LF-to-VD process to obtain a molten steel with qualified composition. A four-hole immersion nozzle was used, the casting cross-section was 325 mm × 380 mm, the tundish superheat was 26°C, the casting speed was 0.6 m / min, and the secondary cooling water ratio was 0.24 L / kg.

[0064] Determination of main technical process parameters:

[0065] Offline measurements of the magnetic flux density at the center of the crystallizer and the electromagnetic stirrer at the end of solidification were conducted using a Lakeshore 475DS gaussmeter in alternating low-frequency mode and a TM801 gaussmeter in alternating mode, respectively, at different stirring currents and frequencies. Table 3 (only partial data) shows that the magnetic flux density at the center of the crystallizer and the end of solidification measured with the TM801 gaussmeter was approximately one-fold lower than that measured with the Lakeshore 475DS gaussmeter. Furthermore, the relationships between magnetic flux density, stirring current, and frequency did not conform to conventional estimates, making it difficult to optimize and determine the optimal stirring frequency.

[0066] Table 3 Magnetic induction intensity of the mold and electromagnetic stirrer at the end of solidification measured by different Gauss meters (Gs)

[0067]

[0068] The optimal stirring frequency of the electromagnetic stirrer was calculated using formula (1) based on the central magnetic induction intensity of the crystallizer and the electromagnetic stirrer at different currents and frequencies measured by a Lakeshore 475DS Gaussmeter. The optimal stirring frequency was 2.5 Hz for a crystallizer electromagnetic stirring current of 100 A, a stirrer central magnetic induction intensity of 125 Gs, and a stirrer frequency of 2.5 Hz for a stirrer at the end of solidification current of 250 A, a stirring mode of alternating 10 s-2 s-10 s, a stirrer central magnetic induction intensity of 300 Gs, and a stirrer frequency of 6 Hz for a stirrer at the end of solidification current of 250 A.

[0069] The solidification heat transfer model, validated by temperature measurement and nail-shooting tests, was used to calculate the center solid fraction of the ingot. The center solid fraction was then used to determine the installation location of the electromagnetic stirrer at the end of solidification, as well as the locations and reductions for light and heavy reduction. The electromagnetic stirrer was installed at a position 15.7 m corresponding to a center solid fraction of 0.12. Soft reduction was performed using the 2# and 3# straightening machines for the center solid fractions of 0.3-0.48, with a total reduction of 5 mm. For the center solid fraction of 0.9-1, heavy reduction was performed using the 4# straightening machine for the center solid fraction, with a total reduction of 5 mm. Table 4 shows the center solid fractions and reductions for different straightening machines.

[0070] Table 4 Reduction amount of light and heavy pressure of different tension levelers

[0071]

[0072] Production preparation:

[0073] After the continuous casting machine was started, continuous casting was carried out according to the determined casting machine process parameters. Medium carbon alloy steel 42CrMoA was cast by BOF→LF→VD to obtain molten steel with qualified composition. A four-hole immersion nozzle was used, the casting section was 325mm×380mm, the tundish superheat was 26°C, the casting speed was 0.6m / min, and the secondary cooling water volume was 0.24L / Kg. The crystallizer electromagnetic stirring current was 100A, the magnetic induction intensity at the center of the stirrer was 125Gs, and the optimal stirring frequency was 2.5Hz. The electromagnetic stirring at the end of solidification was installed at 15.7m, the electromagnetic stirring current at the end of solidification was 250A, the stirring mode was alternating 10s-2s-10s, the magnetic induction intensity at the center of the stirrer was 300Gs, and the optimal stirring frequency was 6Hz. The tension and leveling machine was operated with light reduction and heavy reduction as shown in Table 4.

[0074] The cross section of the ingot obtained in this embodiment was scanned and analyzed by the metal in-situ scanner OPA. The carbon composition distribution of the cross section is as follows: Figure 5 As shown, the cross-sectional density distribution obtained by in-situ analysis of the in-situ scanner OPA is as follows: Figure 6 As shown; the carbon composition distribution of the inner and outer arc center lines of the cross section of the ingot obtained by drilling is as follows Figure 7 As shown, the cross section of the ingot prepared in this embodiment has no shrinkage cavity in the center, the central equiaxed crystal ratio is 28%, the columnar crystal ratio is 62%, and the mixed crystal region ratio is 10%. Figure 5 and Figure 6 As shown in the OPA scan, the carbon content of the ingot in this embodiment is highest in the area 1 / 2 to 3 / 4 from the center, the extreme difference in carbon content in the cross section is 0.04%, and the center density is 97%. Figure 7 As shown, the maximum carbon content of the cross section of the ingot in this embodiment is also in the area 1 / 2 to 3 / 4 from the center, which is about 0.44%. The pattern is consistent with the results of OPA scanning, and the cross-section carbon range is 0.03%.

[0075] Example 2

[0076] This embodiment also provides a highly homogenized medium-low carbon alloy steel large-section rectangular billet and its preparation method. The basic preparation process is the same as that of Example 1, except that a different type of medium-low carbon alloy steel and cross-section are used. The steel grade is 20CrMnTiH, and the cross-section is 390mm×510mm. The specific composition of the steel grade is shown in Table 5:

[0077] Table 5 Main chemical composition of low carbon alloy steel 20CrMnTiH (wt%)

[0078]

[0079] The preparation method of this embodiment is basically the same as that of Example 1, except that:

[0080] During the equipment optimization phase, 10 tension and leveling machines were set up;

[0081] The magnetic induction intensity of the electromagnetic stirrer set at the crystallizer and the end of solidification was measured offline using a Lakeshore 475DS Gauss meter. The electromagnetic stirring current of the crystallizer was 150A, the stirring frequency was 1.5Hz, and the magnetic induction intensity at the center of the stirrer was 125Gs. The electromagnetic stirring current at the end of solidification was 150A, the stirring frequency was 8Hz, the stirring mode was alternating 10s-5s-10s, and the magnetic induction intensity at the center of the stirrer was 172Gs.

[0082] The corresponding solid phase ratio at the center of the ingot is 0.18, and the electromagnetic stirring installation position at the end of solidification is 11.47m;

[0083] The measured reduction positions and single-roll reduction amounts under light and heavy reduction of the 10 tension levelers are shown in Table 6.

[0084] Table 6 Reduction amount of light and heavy pressure of different tension levelers

[0085]

[0086] In the preparation stage, the molten steel with qualified composition is cast through BOF→LF→RH smelting. The submerged nozzle adopts a four-hole nozzle, the casting section is 390mm×510mm, the superheat of the ladle is 28℃, the pulling speed is 0.41m / min, the secondary cooling water volume is 0.13L / Kg, the electromagnetic stirring current of the crystallizer is 150A, the stirring frequency is 1.5Hz, the magnetic induction intensity at the center of the stirrer is 125Gs, the electromagnetic stirring current at the end of solidification is 150A, the stirring frequency is 8Hz, the stirring mode is alternating 10s-5s-10s, the magnetic induction intensity at the center of the stirrer is 172Gs, and the light pressure and heavy pressure operations are performed on the straightening machine as shown in Table 6.

[0087] The cross-sectional carbon composition distribution of the ingot obtained in this embodiment is obtained by scanning and analyzing the in-situ metal analyzer OPA. Figure 8 As shown, the cross-section density distribution of the ingot is obtained by scanning the metal in-situ analyzer OPA. Figure 9 As shown in the figure, the carbon composition distribution of the inner and outer arc center lines of the cross section of the ingot obtained by drilling is as follows Figure 10 As shown, the cross-section of the ingot prepared in this embodiment has no shrinkage cavity in the center, the central equiaxed crystal ratio is 34%, the columnar crystal ratio is 58%, and the mixed crystal area ratio is 8%. Figure 8 and Figure 9 As shown in the OPA scan, the carbon content of the ingot in this embodiment is highest in the area 1 / 2 to 3 / 4 from the center, the extreme difference in carbon content in the cross section is 0.04%, and the density in the center is 98%. Figure 10As shown, the maximum carbon content in the cross section of the ingot in this embodiment is also in the area 1 / 2 to 3 / 4 from the center, which is about 0.03%. The pattern is consistent with the results of OPA scanning, and the cross-section carbon range is 0.03%.

[0088] Comparative Example

[0089] This comparative example also provides a medium-low carbon alloy steel large-section rectangular billet and preparation method. Compared with Example 1, the parameters of electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are different, and other parameters are the same. The same steel grade 42CrMoA is used, and the steel liquid with qualified composition is smelted by BOF→LF→VD for casting. The submerged nozzle adopts a four-hole nozzle, the casting section is 325mm×380mm, the superheat of the ladle is 23℃, the pulling speed is 0.6m / min, the secondary cooling water volume is 0.26L / Kg, the electromagnetic stirring current of the crystallizer is 450A, the optimal stirring frequency is 3Hz, the magnetic induction intensity of the stirrer center is 235Gs, the electromagnetic stirring installation position at the end of solidification is 15.7m, and the end of solidification is 15.7m. The electromagnetic stirring current is 300A, the optimal stirring frequency is 7Hz, the stirring mode is alternating 10s-2s-10s, the magnetic induction intensity at the center of the stirrer is 400Gs, and the corresponding solid phase ratio at the center of the billet is 0.03. The solid phase ratio and reduction amount at the center of the billet corresponding to different drawing and straightening machines are shown in Table 7. Among them, the solid phase ratio at the center of the billet of 2#~4# drawing and straightening machines is in the range of 0.27~0.76, and light reduction is performed, with a total reduction of 8mm. The solid phase ratio at the center of the billet of 5# drawing and straightening machine is in the range of 1, and heavy reduction is performed, with a total reduction of 5mm.

[0090] Table 7 Reduction amount of light and heavy pressure of different tension levelers

[0091]

[0092] The cross section of the ingot obtained in this comparative example was analyzed by metal in-situ analyzer OPA scanning to obtain the carbon composition distribution of the cross section. Figure 11 As shown, the cross-section density distribution of the ingot is obtained by scanning the metal in-situ analyzer OPA. Figure 12 As shown in the figure, the carbon composition distribution of the inner and outer arc center lines of the cross section of the ingot obtained by drilling is as follows Figure 13 As shown. The prepared ingot has no shrinkage cavity in the center of the cross section, the central equiaxed crystal ratio is 37%, the columnar crystal ratio is 48%, and the mixed crystal area ratio is 25%. Figure 11 and Figure 12 As shown in the figure, the carbon content of the in-situ metal analyzer OPA scanning of the ingot cross section is the highest in the area 1 / 2 to 3 / 4 from the center, the extreme difference of the cross section carbon content reaches 0.08%, and the center density is 93%. Figure 13 As shown in the figure, the maximum carbon content in the cross section of the ingot is also in the area 1 / 2 to 3 / 4 from the center, which is about 0.46%. The pattern is consistent with the results of OPA scanning, and the carbon range of the cross section reaches 0.06%.

[0093] From the results of the corresponding low-power and cross-sectional carbon composition distribution of the ingot and the carbon segregation index of the center line of the inner and outer arcs of the cross section in the embodiment and the comparative example, it can be found that by adopting the continuous casting process of the embodiment, the proportion of columnar crystals in the cross section of the ingot increases, the proportion of mixed crystal area decreases, the cross-sectional carbon composition distribution is more uniform, the deviation of carbon composition of the center line of the inner and outer arcs of the cross section decreases, and the frame segregation and cross-sectional carbon extreme difference are significantly improved. In addition, compared with the comparative example, in Example 1, the columnar crystals of the ingot increased from 48% to 62%, the proportion of mixed crystal area decreased from 25% to 10%, the carbon composition extreme difference of the ingot OPA scanning cross section decreased from 0.08% to 0.04%, and the corresponding carbon composition extreme difference analyzed by the drilling carbon-sulfur instrument decreased from 0.06% to 0.03%. By implementing one example and comparing it with a comparative example, both employed mold electromagnetic stirring, end-of-solidification electromagnetic stirring, light reduction, and heavy reduction technologies. While the pulling speeds were the same, the different water ratios in the secondary cooling resulted in different reduction straightening rolls and reductions in the light reduction, and different reduction straightening rolls in the heavy reduction. However, the improvements in center segregation and density were essentially the same for a light reduction of 5-8 mm and a heavy reduction of 5 mm. In the comparative example, due to the high currents in the mold and end-of-solidification electromagnetic stirring, and the suboptimal stirring frequency, the strong stirring force resulted in a large proportion of central equiaxed crystals and center. The pulling speed and end-of-solidification electromagnetic stirring position were not properly matched, resulting in negative segregation in the center of the ingot. Ultimately, the thermal stress field (secondary cooling), electromagnetic field (mold electromagnetic stirring and end-of-solidification electromagnetic stirring), and mechanical stress field (light reduction and heavy reduction) failed to achieve multi-field synergy. Compared with the example, the ingot exhibited severe frame segregation and a large cross-sectional carbon range.

[0094] The above description is only a preferred embodiment of the present invention and an explanation of the technical principles used. It is not intended to limit the scope of the invention to be protected, but merely represents a preferred embodiment of the present invention. Those skilled in the art should understand that the scope of the invention involved in the present invention is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative work shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a highly homogenized medium and low carbon alloy steel large cross-section rectangular billet, characterized in that: The method comprises: Step S1: electromagnetic stirrers are respectively installed at the crystallizer and the solidification end of the continuous casting machine, and 9 to 10 straightening machines are installed at the solidification end; Step S2: Based on the physical equipment including electromagnetic stirring, the solidification heat transfer model and the cellular automaton model are coupled to construct and verify the solidification structure model of medium-carbon low-alloy steel in large-section rectangular billets. With the preset solidification structure of the ingot as the target, the casting speed range, the secondary cooling water ratio range, and the stirring current range of the crystallizer and the electromagnetic stirrer at the end of solidification of the large-section rectangular billet of medium- and low-carbon alloy steel are optimized; Step S3, determining the optimal stirring frequency of the electromagnetic stirrer of the crystallizer; determining the setting position and optimal stirring frequency of the electromagnetic stirrer at the end of solidification; determining the pressing position and single-roll pressing amount of the light reduction and heavy reduction of the straightening machine at the end of solidification; wherein, after the electromagnetic stirrer at the end of solidification, the solid phase ratio at the center of the billet is 0.25~0.85, the straightening machine is set to perform light reduction, and the total amount of light reduction is 5~12mm; the solid phase ratio at the center of the billet is 0.9~1, the straightening machine is set to perform heavy reduction, and the total amount of heavy reduction is 5~6mm; and, The solidification heat transfer model verified by temperature measurement and nail-shooting tests was used to calculate the center solid fraction of the ingot. The location of the electromagnetic stirrer at the end of solidification, the reduction positions under light and heavy pressure, and the corresponding single-roll reduction were determined based on the center solid fraction of the ingot. The center solid fraction of the ingot at the location where the electromagnetic stirrer was installed was 0.1-0.

2. When determining the single-roll reduction under light pressure, the following conditions were used: when the center solid fraction of the ingot was in the range of 0.25-0.5, the single-roll reduction was 1-3 mm; when the center solid fraction was in the range of 0.5-0.85, the single-roll reduction was 3-4 mm, and the total light pressure reduction was 5-12 mm. Step S4, after the continuous casting machine starts casting, in the stage from the tundish to the crystallizer: the molten medium- and low-carbon alloy steel flows from the tundish of the multi-stream large-section rectangular billet continuous casting machine into the crystallizer through the submerged nozzle; Step S5, from the crystallizer to the end of solidification: the crystallizer electromagnetic stirrer rotates and stirs the molten steel, and solidifies it in the secondary cooling zone by mist cooling; at the set position of the solidification end, the solidification end electromagnetic stirrer stirs the molten steel in the core mushy zone; Step S6, from the end of solidification to the stage of casting forming: the casting containing the liquid core that is not completely solidified is bent and straightened by multiple straightening machines; according to the set light reduction position and the corresponding single-roll reduction amount, light reduction is performed at the straightening machine corresponding to the light reduction position with the corresponding light reduction single-roll reduction amount; when approaching the end of solidification, according to the set heavy reduction position and the corresponding single-roll reduction amount, heavy reduction is performed at the straightening machine corresponding to the heavy reduction position with the corresponding heavy reduction single-roll reduction amount; Step S7, for the obtained medium-low carbon alloy steel large-section rectangular billet, the performance evaluation is performed by using the low-magnification morphology and rating of the billet cross section and longitudinal section, the carbon composition of the cross section drilling, the metal in-situ analysis of the cross section carbon composition distribution and the quantitative central density.

2. The preparation method according to claim 1, characterized in that In step S1, the 9 to 10 straightening machines set up all have a light pressure function, and the maximum pressure reduction of a single roller of the straightening machine implementing light pressure is not less than 5 mm. Among them, 4 to 5 straightening machines have a heavy pressure function, and the maximum pressure reduction of a single roller of the straightening machine implementing heavy pressure is not less than 10 mm, and the maximum pressure is 2500KN.

3. The preparation method according to claim 1, characterized in that The mass percentages of the elements in the medium-low carbon alloy steel are C: 0.2%~0.45%, Mn: 0.5%~1.0%, Cr: 0.8%~1.1%, Mo: 0.05%~0.2%, and Ti: 0.01%~0.1%.

4. The preparation method according to claim 1, characterized in that In step S3, determining the optimal stirring frequency includes: Step S301, using a Gauss meter with an alternating low-frequency mode to measure the central magnetic induction intensity of the crystallizer electromagnetic stirrer and the solidification end electromagnetic stirrer in an alternating low-frequency mode offline, and a copper tube is required when measuring the crystallizer electromagnetic stirrer; Step S302, calculating the optimal stirring frequencies of the crystallizer electromagnetic stirrer and the solidification end electromagnetic stirrer according to the central magnetic induction intensity; Step S303 , determining the stirring current of the crystallizer electromagnetic stirrer based on the central magnetic induction intensity of the crystallizer electromagnetic stirrer being 100 Gs to 200 Gs, and determining the stirring current of the solidification end electromagnetic stirrer based on the central magnetic induction intensity of the solidification end electromagnetic stirrer being 200 Gs to 300 Gs.

5. The preparation method according to claim 4, characterized in that When the optimal stirring frequency is calculated using the measured central magnetic induction intensity, the optimal stirring frequency is determined by the following formula: （1） In formula (1), T is the electromagnetic torque, σ is the conductivity of the conductor; f is the stirring frequency; B is the measured magnetic induction intensity at the center of the stirrer; r1 is the equivalent radius of the liquid core; L is the effective radius of the stirrer; The frequency corresponding to the maximum electromagnetic torque is the optimal stirring frequency.

6. The preparation method according to claim 1, characterized in that After the electromagnetic stirrer at the end of solidification, a heavy pressure pull-down leveler is set up near the last light pressure pull-down leveler.

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