Highly homogenized steel and process for its production

Abstract

The present application relates to the field of steelmaking, to solve the problem of high segregation degree of the casting blank when the prior art improves the equiaxed crystal rate of the casting blank by low superheat, and provides a production process of high homogenization steel, comprising: the molten steel passes through a continuous casting crystallizer to obtain a casting blank; the continuous casting crystallizer comprises a crystallization cylinder and a cooling cylinder, the cooling cylinder is sleeved on the crystallization cylinder; the wall thickness of the crystallization cylinder gradually decreases during extending along the flow direction of the molten steel; the relationship between the wall thickness change rate, the superheat and the equiaxed crystal rate is obtained to obtain a first set of pairs; the segregation index of the casting blank under the first set of pairs is obtained to obtain a second set of pairs; the second set of pairs comprises the wall thickness change rate and the superheat; the superheat of the molten steel and the wall thickness change rate meet the second set of pairs. The present application increases the weak point of the columnar crystal structure of the crystallizer with a specific structure, promotes the occurrence of the breaking phenomenon, promotes the increase of the free crystal nucleus, and reduces the segregation degree under the condition of ensuring the equiaxed crystal rate.

Description

Technical Field

[0001] This invention relates to the field of steelmaking, and more specifically, to a highly homogeneous steel and its production process. Background Technology

[0002] 20CrMnTiH, as one of the most widely used carburizing gear steels in China, is a key material for manufacturing high-end automotive gears due to its excellent hardenability, carburizing performance, and strength-toughness balance. However, this steel contains high levels of Mn, Cr, and other elements that expand the austenite region, making it prone to severe dendritic segregation during continuous casting solidification, resulting in uneven composition of the cast billet cross-section. This micro-segregation is inherited and amplified during subsequent hot rolling, forming primary and secondary banded structures, causing uneven carburized layer depth, increased quenching distortion, and in severe cases, decreased gear meshing accuracy and excessive transmission noise.

[0003] The hardenability bandwidth of gear steel is a core indicator for measuring its quality uniformity. An excessively wide hardenability bandwidth, such as >7HRC, means significant differences in the quenched microstructure and properties at different locations within the same batch or even the same ingot. This directly leads to irregular deformation of the gears after heat treatment, making precision assembly impossible. The stability of the hardenability bandwidth fundamentally depends on the compositional uniformity of the steel, i.e., the degree of segregation in the ingot. Therefore, achieving highly homogeneous production of gear steel hinges on overcoming two major technical bottlenecks: solidification control of low-segregation ingots and rolling regulation of low-banding microstructure.

[0004] Existing technologies that improve equiaxed crystal ratio by using low superheat do not necessarily reduce the degree of segregation in the cast billet or reduce it to a poor degree, requiring further processing in subsequent processes, such as high-temperature diffusion processes with longer temperatures and durations, which are costly. Summary of the Invention

[0005] The purpose of this invention is to provide a highly homogeneous steel and its production process, which solves the problem of high segregation in the billet when the existing technology increases the equiaxed crystal ratio of the billet by low superheat.

[0006] The embodiments of the present invention are achieved through the following technical solutions:

[0007] A production process for highly homogeneous steel includes:

[0008] Molten steel is passed through a continuous casting mold to produce a billet; the continuous casting mold includes a crystallization cylinder and a cooling cylinder, the cooling cylinder being fitted over the crystallization cylinder, and a cooling water channel being formed between the cooling cylinder and the crystallization cylinder; a molten steel channel is formed inside the crystallization cylinder; the wall thickness of the crystallization cylinder gradually decreases as it extends along the direction of molten steel flow.

[0009] When the cooling temperature difference is constant, the relationship between the wall thickness change rate, superheat and equiaxed crystal ratio is obtained to obtain the first set of pairs of numbers whose equiaxed crystal ratio meets the preset range; the first pair of numbers includes: wall thickness change rate and superheat; wall thickness change rate = 100% * maximum wall thickness difference / crystallization cylinder length;

[0010] Under the parameters of the first set of ordered pairs, the segregation index of the billet is obtained, and a second set of ordered pairs with a segregation index ≤ segregation threshold is obtained; the second set of ordered pairs includes: wall thickness change rate and superheat.

[0011] The superheat of the molten steel and the rate of change of the wall thickness of the continuous casting mold conform to the second set of numbers.

[0012] 20CrMnTiH, as one of the most widely used carburizing gear steels in China, is a key material for manufacturing high-end automotive gears due to its excellent hardenability, carburizing performance, and strength-toughness balance. However, this steel contains high levels of Mn, Cr, and other elements that expand the austenite region, making it prone to severe dendritic segregation during continuous casting solidification, resulting in uneven composition of the cast billet cross-section. This micro-segregation is inherited and amplified during subsequent hot rolling, forming primary and secondary banded structures, causing uneven carburized layer depth, increased quenching distortion, and in severe cases, decreased gear meshing accuracy and excessive transmission noise.

[0013] Continuous casting involves continuously pouring molten steel into a bottom-opening mold. Once the steel has initially solidified in the mold to form a shell of a certain thickness, a pulling mechanism continuously pulls the shell out from the bottom while new molten steel is continuously poured in from above, ensuring uninterrupted production. Throughout the process, heat is forcibly dissipated from the center of the billet towards the surface, along the pulling direction, resulting in sequential solidification from the outside in and from the bottom up. After being pulled out of the mold with its liquid core, the billet enters a secondary cooling zone, where the shell continues to thicken until the center is completely solidified.

[0014] In continuous casting, the superheat of molten steel, electromagnetic stirring parameters, and cooling intensity all affect the degree of segregation in the cast billet. During the solidification process of molten steel, columnar and equiaxed crystals form. The bridging effect of columnar crystals obstructs the flow of molten steel during subsequent solidification shrinkage, preventing the effective filling of pores created by phase transformation shrinkage. This leads to porosity in the center, enrichment of solute elements such as C, Mn, P, and S, resulting in severe central segregation. Therefore, it is generally necessary to control process parameters to reduce the formation of columnar crystals and increase the equiaxed crystal ratio. When the superheat is too high, the required nucleation undercooling at the beginning of solidification is more difficult to achieve within the melt. Nucleation mainly occurs at the mold wall, leading to the development of columnar crystals. Therefore, low superheat is one of the means to reduce the formation of columnar crystals and increase the equiaxed crystal ratio. However, excessive suppression of columnar crystal formation also has certain drawbacks. For example, when the nucleation core is mainly composed of primary heterogeneous particles, some grains are relatively coarse, increasing the difficulty or power consumption of subsequent grain refinement. Therefore, the applicant hopes to refine the grains as much as possible while ensuring the equiaxed crystal ratio by controlling the growth of columnar crystals. This invention, through the reasonable combination of columnar crystal growth rate control and electromagnetic stirring parameters, allows columnar crystals to be broken or melted by electromagnetic stirring at a certain growth level, forming free crystal nuclei. The increase of free crystal nuclei can promote the formation of equiaxed crystals. However, when columnar crystals grow excessively, not only may the columnar crystals promote segregation, but excessive growth also increases the electromagnetic field strength required for the process. Furthermore, the electromagnetic field can only be applied when the shell reaches a sufficient thickness. At this point, there may be problems such as the liquid core size being too small, the resistance to molten steel flow increasing, and the stirring effect not being able to effectively affect the solidification front, resulting in limited improvement on central segregation.

[0015] While reducing superheat typically increases the equiaxed crystal ratio, it doesn't necessarily reduce or significantly reduce the segregation of the cast billet. For example, with fixed casting speed and subsequent cooling processes, excessively low superheat weakens the diffusion towards a uniform microstructure, reduces the fluidity of the molten steel, and diminishes the feeding effect. Furthermore, equiaxed crystal regions formed at low superheat often have coarser dendrites and a relatively loose microstructure, meaning larger secondary dendrite spacing. This provides wider channels for the flow of residual liquid phases enriched with solute, potentially increasing the angle of V-shaped segregation and causing segregated elements to concentrate more towards the centerline, thus exacerbating central segregation. Therefore, this invention selects a moderate superheat, providing conditions for columnar crystal growth and promoting effective diffusion of molten steel and enriched elements. The formation of columnar crystals is related to the temperature gradient, and their growth direction is usually opposite to the heat dissipation direction. In order to increase the structural weak points and promote the occurrence of fracture when the columnar crystals can grow to a certain extent, thereby promoting the increase of free crystal nuclei, this invention increases the diversity or disorder of heat dissipation paths through process improvement, thereby promoting the emergence of columnar crystal branching or bending structures. After increasing the inflection point structure, the weak points increase, which is conducive to the fracture of columnar crystals.

[0016] The applicant improved the existing copper plate structure of the crystallizer, making its thickness variable while keeping the widths of the molten steel flow channel and the cooling water flow channel constant. This results in different heat exchange efficiencies at different locations on the copper plate during heat exchange, thereby increasing the temperature gradient along the molten steel flow direction. In this invention, the copper plate thickness at the molten steel outlet is the smallest, resulting in the highest heat exchange efficiency, while the copper plate thickness at the inlet is the largest, resulting in the lowest heat exchange efficiency. This further increases the temperature difference between the molten steel at the inlet and outlet. With a fixed molten steel flow channel, the temperature gradient increases significantly.

[0017] Preferably, the superheat of the molten steel is 20~30℃, the wall thickness variation rate of the crystallizing cylinder is 3%~6%, and the average wall thickness of the crystallizing cylinder is 30~50mm.

[0018] Preferably, the length of the crystallization cylinder is 700~1000mm, and the temperature difference between the inlet and outlet of the cooling cylinder is 6~10℃.

[0019] Preferably, the specific water volume in the secondary cooling zone is 0.3~0.35L / kg, and the billet pulling speed is 1.2~1.4m / min.

[0020] After the molten steel is cooled once in the crystallizer, a shell is formed. The billet pulled out by the casting mechanism consists of the shell and the molten steel surrounding it. After passing through the secondary cooling zone, the unsolidified molten steel continues to cool and solidify until the billet is completely solidified. If the specific water content in the secondary cooling zone is too high, it can lead to insufficient time for the solute elements to diffuse backward in the solid phase, which is not conducive to compositional homogenization. If the specific water content in the secondary cooling zone is too low, it can promote the coarsening of columnar crystals and inhibit the formation of equiaxed crystals. In addition, it can also lead to a slower casting speed, limiting production efficiency.

[0021] The specific water volume is the amount of cooling water consumed per kilogram of steel in the secondary cooling zone.

[0022] Preferably, the production process further includes: primary electromagnetic stirring and secondary electromagnetic stirring; primary electromagnetic stirring is distributed at the first point to the second point, and secondary electromagnetic stirring is distributed at the third point to the fourth point; the distance between the first point and the crystallization cylinder is 120~160mm, the distance between the first point and the second point is 1 / 8~1 / 6 of the length of the crystallization cylinder, the distance between the third point and the second point is 20~30mm, and the distance between the fourth point and the third point is 1 / 6~1 / 4 of the length of the crystallization cylinder.

[0023] Columnar crystals can be broken up by primary and secondary electromagnetic stirring, increasing the disorder of heat dispersion and further reducing the structural strength of the columnar crystals. The reason for using secondary electromagnetic stirring is at least as follows: when the molten steel first enters the crystallizer, the magnetic field strength should not be too high, allowing the columnar crystals some room to grow. Furthermore, the initially formed shell is relatively thin, so an excessively high magnetic field strength is not advisable. After the columnar crystals have grown, secondary electromagnetic stirring breaks them up. The broken dendritic fragments serve as nucleation sites for equiaxed crystals, promoting their formation. In addition, the shell in the region corresponding to the secondary electromagnetic stirring is thicker, making it more suitable for a higher magnetic field strength. If a uniform electromagnetic strength is used in the electromagnetic stirring region in this invention, the electromagnetic strength is affected by the thin initial shell thickness and cannot be too high. In this case, either the distance between the electromagnetic region and the crystallizer inlet must be increased, or the stirring effect in the thicker shell region must be sacrificed. Either way, the breaking effect of the columnar crystals will be reduced, which is detrimental to the formation of equiaxed crystals. Furthermore, ineffective stirring will increase the degree of segregation.

[0024] Preferably, the current for the first electromagnetic stirring is 80~120A and the frequency is 5~6Hz; the current for the second electromagnetic stirring is 150~200A and the frequency is 4~4.5Hz.

[0025] Since the thickness of the crystallizing cylinder in this invention is also changing, and its trend is exactly opposite to that of the billet shell thickness, the frequency of the primary stirring differs from that of the secondary stirring to ensure effective electromagnetic penetration. Generally, the thicker the crystallizing cylinder wall, the lower the frequency used. However, this invention needs to consider the superposition of the shielding effects between the billet shell and the crystallizing cylinder, making it difficult to directly determine that the frequency used for primary stirring is necessarily lower than that for secondary stirring. Experiments showed that when the secondary stirring frequency is close to but slightly lower than the primary stirring frequency, the billet performance is superior.

[0026] The primary and secondary electromagnetic stirring devices can be built into the space between the crystallization cylinder and the cooling cylinder, or they can be sleeved on the outer wall of the crystallizer or the cooling cylinder. The parameters in this invention are more suitable for an externally integrated design. Electromagnetic stirring devices for crystallizers are existing technology and will not be described in detail here.

[0027] Preferably, the production process further includes: end electromagnetic stirring; the diameter of the liquid core in the action area of ​​the end electromagnetic stirring is 1 / 3 to 1 / 2 of the diameter of the billet cross section; the current intensity is 300 to 400A and the frequency is 8 to 10Hz.

[0028] Preferably, after the fully solidified billet passes through a high-temperature diffusion process, a controlled rolling process, and a controlled cooling process, a highly homogeneous steel is obtained; the temperature of the high-temperature diffusion process is 1245~1260℃, and the time is 60~80min.

[0029] The continuous casting process of this invention produces billets with lower segregation, thereby reducing the adaptability of temperature and time in the high-temperature diffusion process and reducing the banded structure level to below 1.5.

[0030] Preferably, the initial rolling temperature of the controlled rolling process is 1020~1050℃, the finishing rolling temperature is 920~980℃, and the final rolling temperature is 860~900℃; the cooling rate of the controlled cooling process is 12~16℃ / min, the holding time is ≥2h, and the temperature of the cast billet after cooling is ≤300℃.

[0031] During the production process, the time for each stage of the controlled rolling process is related to the mill's capacity, the number of passes, and the temperature drop. According to existing technology, within a defined temperature range, the required deformation to roll the billet to the target finished product thickness is achieved; therefore, the rolling time is not specifically limited. The cooling time can be easily obtained based on the cooling rate and the exit billet temperature.

[0032] A highly homogeneous steel produced by the aforementioned manufacturing process, wherein the highly homogeneous steel is used as 20CrMnTiH gear steel.

[0033] The present invention has at least the following beneficial effects:

[0034] This invention increases the structural weaknesses of columnar crystals through a crystallizer with a specific structure, promoting breakage and thus increasing the number of free crystal nuclei. While ensuring equiaxed crystal ratio, it reduces segregation. The invention improves the existing copper plate structure of the crystallizer, allowing its thickness to vary while maintaining the width of the molten steel flow channel and the cooling water flow channel. This results in different heat exchange efficiencies at different locations of the copper plate during heat exchange, increasing the temperature gradient along the molten steel flow direction. In this invention, the copper plate thickness at the molten steel outlet is the smallest, resulting in the highest heat exchange efficiency, while the copper plate thickness at the inlet is the largest, resulting in the lowest heat exchange efficiency. This further increases the temperature difference between the molten steel at the inlet and outlet. With a fixed molten steel flow channel, the temperature gradient increases significantly, increasing the diversity or disorder of the heat dissipation path, thus promoting the formation of branched or bent columnar crystal structures. The increased inflection point structure increases weak points, which is beneficial for columnar crystal breakage. This invention further reduces the segregation of the cast billet while meeting the equiaxed crystal ratio requirement, reducing energy consumption in post-processing steps and lowering production costs. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the structure of a continuous casting crystallizer;

[0037] Figure 2 This is a diagram of the secondary banded microstructure of steel.

[0038] Attached diagram description: 1~ Crystallization cylinder, 2~ Cooling cylinder, 3~ Cooling water channel, 4~ Molten steel channel. Detailed Implementation

[0039] To make the objectives, methods, and advantages of the embodiments of the present invention clearer, the methods in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0040] Example 1: As Figure 1 As shown, a production process for highly homogeneous steel includes: passing molten steel through a continuous casting crystallizer to obtain a cast billet;

[0041] The continuous casting crystallizer includes a crystallizing cylinder and a cooling cylinder, wherein the cooling cylinder is fitted over the crystallizing cylinder, and a cooling water channel is formed between the cooling cylinder and the crystallizing cylinder; a molten steel channel is formed inside the crystallizing cylinder; and the wall thickness of the crystallizing cylinder gradually decreases as it extends along the direction of molten steel flow.

[0042] When the cooling temperature difference is constant, the relationship between the wall thickness change rate, superheat and equiaxed crystal ratio is obtained to obtain the first set of pairs of numbers whose equiaxed crystal ratio meets the preset range; the first pair of numbers includes: wall thickness change rate and superheat; wall thickness change rate = 100% * maximum wall thickness difference / crystallization cylinder length;

[0043] Under the parameters of the first set of ordered pairs, the segregation index of the billet is obtained, and a second set of ordered pairs with a segregation index ≤ segregation threshold is obtained; the second set of ordered pairs includes: wall thickness change rate and superheat.

[0044] The superheat of the molten steel and the rate of change of the wall thickness of the continuous casting mold conform to the second set of numbers.

[0045] In practice, the cross-section of the billet can be set as needed, but it is fixed when obtaining the second set of numbers. For example, the billet has a cross-section of 240mm × 240mm.

[0046] The preset range can be set as needed. For example, the preset range is 45%~60%. Multiple sets of first pairs may satisfy the preset range; a second set of pairs that meet the billet segregation index requirements can be selected from these sets. For example, the second pair with the lowest segregation index in the second set is used as the actual process parameter. The segregation threshold can be set as needed. For example, the segregation threshold is 1.25.

[0047] This embodiment focuses on improving the continuous casting process; other process parameters in the steel production process can adopt existing technologies.

[0048] As an example, the cross-sectional dimensions of the cooling water channels and the molten steel channels remain unchanged.

[0049] As an example, the cooling cylinder is longer than the crystallizing cylinder in the horizontal direction, so that the cooling water channels completely cover the outer wall of the crystallizing cylinder and facilitate the introduction and exit of cooling water. The crystallizing cylinder is made of copper. A shell can also be installed outside the cooling cylinder for protection and insulation.

[0050] Example 2: Based on Example 1, a production process for high homogeneous steel is provided, characterized by including: molten steel passing through a continuous casting crystallizer to obtain a billet; the billet being pulled out of the continuous casting crystallizer by a billet puller and passing through a secondary cooling zone to obtain a fully solidified billet; the fully solidified billet passing through a high-temperature diffusion process, a controlled rolling process, and a controlled cooling process to obtain high homogeneous steel.

[0051] The superheat of the molten steel is 20℃, the wall thickness variation rate of the crystallizing cylinder is 3%, the average wall thickness of the crystallizing cylinder is 20mm, the length of the crystallizing cylinder is 700mm, the inlet and outlet temperature difference of the cooling cylinder is 6℃ (the inlet temperature of the cooling water is 30℃, and the outlet temperature is 36℃), the specific water volume of the secondary cooling zone is 0.3L / kg, and the billet pulling speed is 1.2m / min.

[0052] The production process also includes: primary electromagnetic stirring, secondary electromagnetic stirring, and final electromagnetic stirring. Primary electromagnetic stirring is distributed between points one and two, and secondary electromagnetic stirring is distributed between points three and four. The distance between point one and the crystallizing cylinder is 120 mm; the distance between point one and point two is 1 / 8 of the crystallizing cylinder length; the distance between point three and point two is 20 mm; and the distance between point four and point three is 1 / 6 of the crystallizing cylinder length. The current for primary electromagnetic stirring is 80 A, and the frequency is 4 Hz; the current for secondary electromagnetic stirring is 150 A, and the frequency is 5 Hz. In the area of ​​final electromagnetic stirring, the liquid core diameter occupies 1 / 3 to 1 / 2 of the billet cross-sectional diameter; the current intensity is 300 A, and the frequency is 8 Hz.

[0053] The high-temperature diffusion process has a temperature of 1245℃ and a time of 60 min; the controlled rolling process has an initial rolling temperature of 1020℃; a finishing rolling temperature of 920℃ and a final rolling temperature of 860℃; the controlled cooling process has a cooling rate of 12℃ / min, a holding time of 2.5 h, and a final temperature of 300℃ after the billet has cooled.

[0054] Example 3: Based on Example 1, a production process for high homogeneous steel is provided, characterized by including: molten steel passing through a continuous casting crystallizer to obtain a billet; the billet being pulled out of the continuous casting crystallizer by a billet puller and passing through a secondary cooling zone to obtain a fully solidified billet; the fully solidified billet passing through a high-temperature diffusion process, a controlled rolling process, and a controlled cooling process to obtain high homogeneous steel.

[0055] The superheat of the molten steel is 30℃, the wall thickness variation rate of the crystallizing cylinder is 6%, the average wall thickness of the crystallizing cylinder is 40mm, the length of the crystallizing cylinder is 1000mm, the inlet and outlet temperature difference of the cooling cylinder is 10℃ (the inlet temperature of the cooling water is 30℃, and the outlet temperature is 40℃), the specific water volume of the secondary cooling zone is 0.35L / kg, and the billet drawing speed is 1.4m / min.

[0056] The production process also includes: primary electromagnetic stirring, secondary electromagnetic stirring, and final electromagnetic stirring. Primary electromagnetic stirring is distributed between points one and two, and secondary electromagnetic stirring is distributed between points three and four. The distance between point one and the crystallizing cylinder is 160 mm; the distance between point one and point two is 1 / 6 of the crystallizing cylinder length; the distance between point three and point two is 30 mm; and the distance between point four and point three is 1 / 4 of the crystallizing cylinder length. The current for primary electromagnetic stirring is 120 A, and the frequency is 4.5 Hz; the current for secondary electromagnetic stirring is 200 A, and the frequency is 6 Hz. In the area of ​​final electromagnetic stirring, the liquid core diameter occupies 1 / 3 to 1 / 2 of the billet cross-sectional diameter; the current intensity is 400 A, and the frequency is 10 Hz.

[0057] The high-temperature diffusion process has a temperature of 1260℃ and a time of 80 min; the controlled rolling process has an initial rolling temperature of 1050℃, a finishing rolling temperature of 920℃, and a final rolling temperature of 900℃; the controlled cooling process has a cooling rate of 16℃ / min, a holding time of 2.5 h, and a final temperature of 280℃ after the billet has cooled.

[0058] Example 4: Based on Example 1, a production process for high homogeneous steel is provided, characterized by including: molten steel passing through a continuous casting crystallizer to obtain a billet; the billet being pulled out of the continuous casting crystallizer by a billet puller and passing through a secondary cooling zone to obtain a fully solidified billet; the fully solidified billet passing through a high-temperature diffusion process, a controlled rolling process, and a controlled cooling process to obtain high homogeneous steel.

[0059] The superheat of the molten steel is 25℃, the wall thickness variation rate of the crystallizing cylinder is 4.5%, the average wall thickness of the crystallizing cylinder is 30mm, the length of the crystallizing cylinder is 850mm, the inlet and outlet temperature difference of the cooling cylinder is 8℃ (the inlet temperature of the cooling water is 30℃, and the outlet temperature is 38℃), the specific water volume of the secondary cooling zone is 0.32L / kg, and the billet pulling speed is 1.3m / min.

[0060] The production process also includes: primary electromagnetic stirring, secondary electromagnetic stirring, and final electromagnetic stirring. Primary electromagnetic stirring is distributed between the first and second points, and secondary electromagnetic stirring is distributed between the third and fourth points. The distance between the first point and the crystallizing cylinder is 140 mm; the distance between the first and second points is 1 / 7 of the crystallizing cylinder length; the distance between the third and second points is 25 mm; and the distance between the fourth and third points is 1 / 5 of the crystallizing cylinder length. The current for primary electromagnetic stirring is 100 A, and the frequency is 4.2 Hz; the current for secondary electromagnetic stirring is 180 A, and the frequency is 5.5 Hz. In the final electromagnetic stirring area, the liquid core diameter occupies 1 / 3 to 1 / 2 of the billet cross-sectional diameter; the current intensity is 350 A, and the frequency is 9 Hz.

[0061] The high-temperature diffusion process has a temperature of 1250℃ and a time of 70 min; the controlled rolling process has an initial rolling temperature of 1035℃, a finishing rolling temperature of 950℃, and a final rolling temperature of 880℃; the controlled cooling process has a cooling rate of 14℃ / min, a holding time of 2.5 h, and a final temperature of 280℃ after the billet has cooled.

[0062] Example 5: The difference from Example 4 is that there is no primary electromagnetic stirring or secondary electromagnetic stirring.

[0063] Comparative Example 1: The difference from Example 4 is that there was no electromagnetic stirring.

[0064] Comparative Example 2: The difference from Example 4 is that there is no secondary electromagnetic stirring.

[0065] Comparative Example 3: The difference from Example 4 is that the current for both the first and second electromagnetic stirring is 180A.

[0066] Comparative Example 4: The difference from Example 4 is that the frequency of both the first and second electromagnetic stirring is 5.5 Hz.

[0067] Comparative Example 5: The difference from Example 4 is that the wall thickness of the crystallization cylinder remains unchanged at 30 mm.

[0068] Comparative Example 6: The difference from Example 4 is that the temperature of the high-temperature diffusion process is 1250°C and the time is 30 minutes.

[0069] Experiment: The composition of the molten steel is shown in Table 1. The cross-section of the billet is 240mm×240mm.

[0070] Table 1

[0071]

[0072] Completely solidified billets were prepared according to the production processes provided in Examples 2-5 and Comparative Examples 1-5. The equiaxed crystal ratio and central carbon segregation index (C / C0) of the billets were tested. All electromagnetic stirring was carried out in an alternating manner: forward stirring for 20 seconds, pause for 5 seconds, and reverse stirring for 20 seconds. After multiple sets of tests, the average value was taken. The test results are shown in Table 2.

[0073] Table 2

[0074]

[0075] As can be seen from the experimental data of Examples 2-5, the cast billet produced by the production process provided by the present invention has better quality, and the degree of segregation is reduced while ensuring the equiaxed crystal ratio.

[0076] A comparison of the results from Comparative Examples 1-2 and Example 4 shows that both electromagnetic stirring operations at the crystallizer affected the quality of the cast billet. Without secondary electromagnetic stirring, the carbon segregation index was below 1.0, indicating negative segregation, possibly due to the formation of regions with excessively low composition caused by the lack of stirring. The decrease in equiaxed crystal ratio may be related to the strength and fracture effect of the columnar crystal structure.

[0077] A comparison of the results from Comparative Examples 3-4 and Example 4 shows that the magnitude and frequency of the current during electromagnetic stirring also affect the quality of the cast billet. Excessive current during a single electromagnetic stirring operation may limit the initial formation of columnar crystals, and conversely, affect the formation of equiaxed crystals to some extent. Conversely, excessively high frequency during a single electromagnetic stirring operation may cause excessive attenuation of the electromagnetic flux as it passes through the thicker walls of the crystallization cylinder, making effective stirring difficult.

[0078] A comparison of the results of Comparative Example 5 and Example 4 shows that the quality of the cast billet is significantly improved by using a variable crystallizer wall thickness in this invention.

[0079] Highly homogeneous steels were prepared using the production processes provided in Examples 2-5 and Comparative Examples 1-6, and the hardenability bandwidth (HRC) was tested. After multiple sets of tests, the average value was taken, and the test results are shown in Table 3.

[0080] Table 3

[0081]

[0082] As can be seen from the data in the table, the steel produced using the production process provided by this invention exhibits good quality uniformity. The experimental results of Comparative Examples 4 and 6 show that the high-temperature diffusion process in this invention consumes less energy, while the performance of the steel obtained is comparable to that of Comparative Example 6.

[0083] Figure 2 The image shows the secondary banded microstructure of the steel produced according to the production processes provided in Example 4 and Comparative Example 6.

[0084] The left image is the morphology diagram corresponding to Comparative Example 6, and the right image is the morphology diagram corresponding to Example 4. A comparison of the two images shows that the contrast between the bright and dark bands in the right image is significantly reduced, proving that the banding level is significantly reduced after using a crystallization cylinder with varying wall thickness.

[0085] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A production process for highly homogeneous steel, characterized in that, include: Molten steel is passed through a continuous casting mold to produce a billet; The continuous casting crystallizer includes a crystallizing cylinder and a cooling cylinder, wherein the cooling cylinder is fitted over the crystallizing cylinder, and a cooling water channel is formed between the cooling cylinder and the crystallizing cylinder; a molten steel channel is formed inside the crystallizing cylinder; and the wall thickness of the crystallizing cylinder gradually decreases as it extends along the direction of molten steel flow. When the cooling temperature difference is constant, the relationship between the wall thickness change rate, superheat and equiaxed crystal ratio is obtained to obtain the first set of pairs of numbers whose equiaxed crystal ratio meets the preset range; the first pair of numbers includes: wall thickness change rate and superheat; wall thickness change rate = 100% * maximum wall thickness difference / crystallization cylinder length; Under the parameters of the first set of ordered pairs, the segregation index of the billet is obtained, and a second set of ordered pairs with a segregation index ≤ segregation threshold is obtained; the second set of ordered pairs includes: wall thickness change rate and superheat. The superheat of the molten steel and the rate of change of the wall thickness of the continuous casting mold conform to the second set of numbers.

2. The production process according to claim 1, characterized in that, The superheat of the molten steel is 20~30℃, the wall thickness variation rate of the crystallizer is 3%~6%, and the average wall thickness of the crystallizer is 20~40mm.

3. The production process according to claim 1, characterized in that, The length of the crystallization cylinder is 700~1000mm, and the temperature difference between the inlet and outlet of the cooling cylinder is 6~10℃.

4. The production process according to claim 1, characterized in that, The specific water volume in the secondary cooling zone is 0.3~0.35L / kg, and the billet pulling speed is 1.2~1.4m / min.

5. The production process according to any one of claims 1 to 4, characterized in that, include: The production process also includes: primary electromagnetic stirring and secondary electromagnetic stirring; primary electromagnetic stirring is distributed from the first point to the second point, and secondary electromagnetic stirring is distributed from the third point to the fourth point; the distance between the first point and the crystallization cylinder is 120~160mm, the distance between the first point and the second point is 1 / 8~1 / 6 of the length of the crystallization cylinder, the distance between the third point and the second point is 20~30mm, and the distance between the fourth point and the third point is 1 / 6~1 / 4 of the length of the crystallization cylinder.

6. The production process according to claim 5, characterized in that, The current for primary electromagnetic stirring is 80~120A and the frequency is 4~4.5Hz; the current for secondary electromagnetic stirring is 150~200A and the frequency is 5~6Hz.

7. The production process according to claim 5, characterized in that, It also includes: end electromagnetic stirring; the diameter of the liquid core in the action area of ​​the end electromagnetic stirring is 1 / 3 to 1 / 2 of the diameter of the billet cross section; the current intensity is 300 to 400A and the frequency is 8 to 10Hz.

8. The production process according to claim 5, characterized in that, After complete solidification, the billet undergoes a high-temperature diffusion process, a controlled rolling process, and a controlled cooling process to obtain highly homogeneous steel. The temperature of the high-temperature diffusion process is 1245~1260℃, and the time is 60~80min.

9. The production process according to claim 8, characterized in that, The controlled rolling process has an initial rolling temperature of 1020~1050℃, a finishing rolling temperature of 920~980℃, and a final rolling temperature of 860~900℃; the controlled cooling process has a cooling rate of 12~16℃ / min, a holding time of ≥2h, and a billet temperature of ≤300℃ after cooling.

10. A highly homogeneous steel produced by the production process according to any one of claims 1 to 9, characterized in that, The highly homogeneous steel is used as 20CrMnTiH gear steel.

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