Production process for controlling banded structure of medium carbon steel plate

Abstract

The invention discloses a production process for controlling a banded structure of a medium carbon steel plate, which belongs to the technical field of metallurgy and specifically comprises the following steps: S1, continuous casting: casting at a low superheat degree of 15-25 DEG C during molten steel casting; the specific water flow of a nozzle in a secondary cooling area of continuous casting machine equipment is controlled to be 0.55 L / kg, and electromagnetic stirring roller equipment is arranged at the secondary cooling area equipment of the continuous casting machine to carry out a single-roller low-current stirring process strategy; s2, a rolling procedure: controlling the total in-furnace time of heating furnace equipment to be 9-14 min / cm during rolling, and controlling the heating tapping temperature of a heating furnace to be 1240 + / -20 DEG C; the cooling speed is controlled to be 1-5 DEG C / s during cooling after rolling; according to the process, a secondary cooling area electromagnetic stirring process is adjusted, a reasonable rolling process is formulated, and the inner center segregation condition of a casting blank and the corresponding rolled material banded structure condition are effectively improved.

Description

Technical Field

[0001] This invention relates to a process for controlling the strip structure of steel plates, specifically a production process for controlling the strip structure of medium carbon steel plates, and belongs to the field of metallurgical technology. Background Technology

[0002] Banded defects disrupt the uniformity of steel, causing differences in the microstructure between adjacent bands, which in turn leads to differences in the properties of adjacent bands. These differences cause stress concentration between strong and weak bands. If subjected to external stress, the interface will become a weak area of ​​the steel, reducing the steel's plasticity, impact toughness, fracture toughness, and reduction of area. Furthermore, the area at the interface between the matrix and the banded defect often becomes the origin of defects such as fatigue cracks.

[0003] High homogeneity of continuously cast billets is an important guarantee for the continuous breakthrough and upgrading of new-generation high-end products. Its efficient manufacturing technology, product quality, stability and consistency control technology represent the country's comprehensive competitiveness. In high-end rolled products, the banded structure defects caused by element segregation have an increasingly significant impact on performance. High-end railway steel has high requirements for the banded structure in steel plates because the banded defects in rail transit bogies significantly affect the impact toughness and weldability of the material, resulting in a reduction in product service life. During transportation, vehicle traction and braking will generate instantaneous impact loads. Vehicle structural components are prone to damage or even fracture during service due to long-term instantaneous impact of loads. It is necessary to control the banded structure level of steel plates and improve the performance level of railway steel plates.

[0004] Currently, patent CN109022732B discloses a production method for reducing banded microstructure in medium- and high-carbon structural steel plates. This method involves controlling the superheat of molten steel to 15-30℃ and the casting speed to 0.8-1.2 m / min. The billet is heated to 1200-1240℃. This reduces the dendritic segregation of easily segregating elements such as C, Mn, and Cr, and also prevents cracks in the billet during casting. If the superheat of the molten steel is too low, the fluidity will be poor, potentially leading to casting interruption due to insufficient flow. If the superheat is above 30℃ or the casting speed is too slow, chemical element segregation will be more severe, hindering the control of banded microstructure. Conversely, if the casting speed is too fast, it can easily cause bulging. This invention can control the banded structure of medium and high carbon structural steel plates to below level 1, and can also improve the plasticity and toughness of the material and increase the fatigue life of the product, thus preventing production accidents such as billet cracks and even steel leakage. However, this method only controls the banded structure from the perspective of affecting the segregation of chemical elements, and the degree of improvement of the banded structure is effective but needs to be improved. However, the lack of low-current electromagnetic stirring in the secondary cooling zone of the continuous casting machine results in a large distribution range of banded segregation, large point segregation size, and high enrichment of elements, which means that the quality of the final product needs to be improved.

[0005] Currently, patent CN113088654B discloses a production method for improving macrosegregation in niobium-ferritic stainless steel plates. It describes inputting molten steel into a continuous casting machine, setting the superheat to 10-25℃, and the secondary cooling water strength to 1.0-1.3 L / kg. Electromagnetic stirring is used in the secondary cooling zone of the continuous casting machine, with a stirring current set to 1500-1800 A. The resulting continuous casting billet is then held at 1200-1250℃ for 4-6 hours before being fed into a hot rolling mill. This process involves continuous casting… The combined control of the three processes of "hot rolling + cold rolling" optimizes the solidification process of niobium-containing ferritic stainless steel continuous casting billets in the smelting process, reducing the macroscopic segregation of niobium; by improving the rolling and annealing processes of hot-rolled steel plates, the distribution of niobium in the steel plates is further made more uniform; the rolling and annealing processes of cold-rolled steel plates are adjusted to regulate the microstructure of niobium-containing ferritic stainless steel plates. However, if the stirring current is too large during continuous casting, it will lead to an increase in the width and number of banded segregation, which will affect the improvement of the final banded structure.

[0006] Therefore, there is an urgent need in this field for a technology that can directly utilize existing medium and heavy plate rolling mills to efficiently and cost-effectively roll large-size, high-performance ribbed steel plates. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a production process for controlling the banded structure of medium carbon steel plates. This process involves adjusting the electromagnetic stirring process in the secondary cooling zone and formulating a reasonable rolling process, which effectively improves the central segregation of the billet and the corresponding banded structure of the rolled material.

[0008] To address the above technical problems, this invention provides a production process for controlling the strip-like structure of medium carbon steel plates, specifically including the following steps: S1, Continuous casting process During the pouring of molten steel, the molten steel in the ladle is poured at a low superheat of 15-25℃; the water flow process parameter of the nozzle in the second cooling zone of the continuous casting machine is controlled at 0.55L / kg; and an electromagnetic stirring roller is set in the second cooling zone of the continuous casting machine to carry out a single-roller low-current stirring process. S2, Rolling process During rolling, the total time in the heating furnace is controlled to be 9-14 min / cm, and the furnace outlet temperature is controlled to be 1240±20℃. After rolling, the cooling rate of the nozzle is controlled to be 1-5℃ / s.

[0009] The technical solution further defined in this invention is: Furthermore, in the aforementioned production process for controlling the strip structure of medium carbon steel plates, the electromagnetic stirring process in the second cooling zone of the continuous casting machine in step S1 adopts a single-roller low-current 160A electromagnetic stirring.

[0010] In the aforementioned production process for controlling the strip structure of medium carbon steel plates, the casting speed parameters required for the continuous casting machine in step S1 are 1.0-1.3 m / min. In the aforementioned production process for controlling the strip structure of medium carbon steel plates, a straight-arc continuous casting machine is used during continuous casting in step S1.

[0011] Technical advantages: This invention employs a straight-arc continuous casting machine, which combines the features of both straight and arc continuous casting machines. It retains the vertical sections of the straight crystallizer and the secondary cooling section, but shortens the dimensions of the vertical sections accordingly and adds a bending section. This causes the billet to bend into an arc shape while still containing a liquid core, and finally straightens through the straightening section. This improves casting speed and billet cleanliness, while overcoming the drawback of inclusion accumulation in the inner arc zone of the billet, thus facilitating the production of high-quality products.

[0012] The beneficial effects of this invention are: This invention minimizes the number of point segregations during the solidification stage. Point segregations only exist in the equiaxed crystal region. The specific water content in conventional continuous casting is generally around 0.45 L / kg, while this invention increases it to 0.55 L / kg. Increasing the cooling intensity and reducing the equiaxed crystal ratio helps to reduce the number of point segregations, increase the proportion of columnar crystals, refine the central equiaxed crystals, and avoid the occurrence of large-sized point segregations, thereby improving the banded structure of the rolled material.

[0013] This invention employs a single-roller low-current battery stirring process. By reducing the electromagnetic stirring current from 320A to 160A, the distribution range of banded segregation is significantly reduced, and the size is also greatly reduced. At the same time, the enrichment of elements is also alleviated. Specifically, the solidification structure and segregation inside the billet are altered, the bright white band of the billet gradually weakens, the proportion of equiaxed crystal regions in the billet decreases from 34.22% to 24.23%, and the size of point segregation decreases, with the average value of the maximum five points decreasing from 4.41 mm2 to 2.95 mm2.

[0014] Heating the billet can homogenize the compositional segregation of the as-cast structure and reduce the deformation resistance during rolling. Therefore, this invention uses a relatively high heating temperature, controlling the furnace exit temperature at 1240±20℃, and also ensuring sufficient heating time with a total furnace time of 9-14 min / cm. This can reduce the banded structure to a certain extent. However, as the heating temperature increases and the heating time increases, problems such as burn-off, overheating, and overburning may occur. Therefore, the heating temperature and heating time are strictly controlled to ensure that problems such as burn-off, overheating, and overburning do not occur while reducing the banded structure.

[0015] Cooling rate is a key factor in improving banded segregation. As the cooling rate increases, the banded structure level is reduced or eliminated. When the post-rolling controlled cooling rate (1~5℃) is greater than the critical cooling rate (1℃ / s), the diffusion of carbon and other elements is suppressed, which can reduce the banded structure level.

[0016] Banded defects disrupt the uniformity of steel, causing differences in the microstructure between adjacent bands, which in turn leads to differences in the properties of adjacent bands. This difference causes stress concentration between strong and weak bands. If subjected to external stress, the interface will become a weak area of ​​the steel, reducing the steel's plasticity, impact toughness, fracture toughness, and reduction of area. This invention improves the effects of center segregation and banded structure by adjusting the electromagnetic stirring process and a reasonable rolling process. Using the continuous casting and rolling processes of this invention, as the electromagnetic stirring current decreases and the size of point segregation decreases, the proportion of banded structure ratings less than 3.0 increases from 33.3% to 80.9%. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of strip segregation of rolled steel under different electromagnetic stirring processes when using the production process of controlling the strip structure of medium carbon steel plate according to the embodiments of the present invention; Figure 2 This is a schematic diagram of the strip structure of rolled steel under different electromagnetic stirring processes when using the production process of controlling the strip structure of medium carbon steel plate according to the embodiments of the present invention. Detailed Implementation

[0018] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings. Example 1

[0019] This embodiment provides a production process for controlling the strip structure of medium carbon steel plates. The main parameters of the casting machine and the produced products include: a straight arc casting machine with an arc radius of 10.0m, a metallurgical length of 31.88m, continuous bending / straightening, and a billet specification of (180mm, 220mm, 260mm) × (1600~2300)mm. The continuous casting machine requires a casting speed of 1.0-1.3m / min. The casting machine has 14 sector sections, and electromagnetic stirring rollers for the secondary cooling zone are installed in the second and third sector sections.

[0020] The above production process, following the normal procedure, involves smelting and continuous casting, heating, rolling, cooling, coiling, and cooling to room temperature, wherein: S1, Continuous casting process During the pouring of molten steel, the molten steel in the ladle is poured at a low superheat of 15-25℃. The specific water volume in the secondary cooling zone of the continuous casting machine is controlled at 0.55 L / kg. Increasing the cooling intensity and reducing the equiaxed crystal ratio helps to reduce the number of point segregations, increase the proportion of columnar crystals, refine the central equiaxed crystals, avoid the occurrence of large-sized point segregations, and thus improve the banded structure of the rolled material. The second cooling zone of the continuous casting machine is equipped with an electromagnetic stirring roller, specifically a single-roller low-current 160A electromagnetic stirring roller. Comparing the banded segregation under the three electromagnetic stirring processes, it can be found that by reducing the electromagnetic stirring current, the distribution range of the banded segregation is significantly reduced, the size is also greatly reduced, and the enrichment degree of elements is also reduced. (1) See Figure 1 As shown, the maximum width of the banded segregation under dual-roller 320A electromagnetic stirring is 95.1 μm. The center exhibits numerous large-sized banded segregations with significant color differences from the surrounding matrix, indicating severe segregation in this region. (See reference...) Figure 2 As shown, the proportion of banded tissue with a rating less than 3.0 was 33.30%; (2) See Figure 1 As shown, the maximum width of the banded segregation under a single-roller 160A electromagnetic stirrer is 34.3 μm. The color difference between the inside and outside of the banded segregation area is small, and the degree of segregation is slight. (See reference...) Figure 2 As shown, the proportion of banded tissue with a rating less than 3.0 was 42.86%; (3) See Figure 1 As shown, the number of banded segregations increased under single-roller 260A electromagnetic stirring, with a maximum width of 63.7 μm. Under high current conditions, the size and number of point segregations increased, while the proportion of large-scale banded structures decreased. (See reference...) Figure 2 As shown, the proportion of banded tissue with a rating less than 3.0 was 11.10%; Depend on Figure 1 and 2 It can be seen that the single-roller 160A electromagnetic stirring process has the fewest banded segregation strips, and no coarse banded segregation was found. The proportion of banded structure rating less than 3.0 increased from 11.10% to 42.86%. Therefore, the single-roller low-current 160A process is adopted. S2, Rolling process Heating the billet can homogenize the compositional segregation of the as-cast structure and reduce the deformation resistance during rolling. Therefore, in this embodiment, the existing heating furnace equipment controls the total furnace time to 9-14 min / cm to ensure sufficient heating time. The furnace exit temperature is set at 1240±20℃ to reduce banded structure. After rolling, the cooling rate is controlled at 2-5℃ / s to increase the cooling rate, suppress the diffusion of carbon and other elements, and reduce the banded structure level.

[0021] This invention describes a practical technical process for controlling banded microstructure in carbon steel plates in the metallurgical industry. The presence of banded microstructure leads to uneven steel structure and severely affects steel properties, reducing plasticity, impact toughness, fracture toughness, and reduction of area. Addressing the problem of banded microstructure defects in rolled steel caused by central element segregation in continuously cast slabs, this invention provides a production process for controlling banded microstructure in steel plates. By optimizing the electromagnetic stirring process in the secondary cooling zone and controlling the rolling heating and cooling processes, effective control of banded microstructure in steel plates is achieved. This improves the first-pass yield of internal quality and performance of steel plates, enhances the market competitiveness of steel plate products, significantly increases economic benefits, and has broad application prospects.

[0022] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A production process for controlling the strip-like structure of medium carbon steel plates, characterized in that, Specifically, the following steps are included: S1, Continuous casting process During the pouring of molten steel, the molten steel in the ladle is poured at a low superheat of 15-25℃; the specific water volume in the secondary cooling zone of the continuous casting machine is controlled at 0.55L / kg, and an electromagnetic stirring roller is installed in the secondary cooling zone of the continuous casting machine to carry out a single-roller low-current stirring process. S2, Rolling process During rolling, the total time in the heating furnace is controlled to be 9-14 min / cm, and the furnace exit temperature is 1240±20℃. After rolling, the cooling rate is controlled to be 1-5℃ / s.

2. The production process for controlling the strip structure of medium carbon steel plates according to claim 1, characterized in that: In step S1, the electromagnetic stirring process of the secondary cooling zone of the continuous casting machine adopts a single-roller low-current 160A electromagnetic stirring.

3. The production process for controlling the strip structure of medium carbon steel plates according to claim 1, characterized in that: The casting speed process parameter required for the continuous casting machine in step S1 is 1.0-1.3 m / min.

4. The production process for controlling the strip structure of medium carbon steel plates according to claim 1, characterized in that: In step S1, a straight-arc continuous casting machine is used during continuous casting.

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