A continuous casting method for improving center segregation of high carbon steel small billets

Abstract

The application belongs to the technical field of continuous casting of steel, and particularly relates to a continuous casting method for improving center segregation of high-carbon steel small billets. The continuous casting method for improving center segregation of high-carbon steel small billets with a size of 160mm*160mm comprises the following steps: after refining, high-carbon steel liquid steel enters a tundish, the superheat degree is controlled to be 17-25 DEG C, and a crystallizer electromagnetic stirring is adopted; the two-cool water control adopts a strategy of separate control of inner and outer arc surfaces and two side surfaces, and the inner and outer arc surface ratio water quantity / two side surface ratio water quantity control range is 2.5-3.5; a dynamic soft reduction process is adopted at the solidification end of the cast billet, the reduction is carried out within a solid phase rate range of 0.2-0.6, and the reduction amount is 10-15mm; and the high-carbon steel is produced according to the continuous casting method, and the cast billet center segregation index is reduced from 1.18 to 1.05 and below.

Description

Technical Field

[0001] This invention belongs to the field of continuous steel casting technology, and in particular to a continuous casting method for improving center segregation of high carbon steel small square billets. Background Technology

[0002] Segregation is an inevitable byproduct of continuous casting, but severe segregation has a significant impact on high-carbon steel. Many methods exist for controlling carbon segregation in billets, including low superheat, electromagnetic stirring, secondary cooling, and light reduction techniques. Among these, end-stage electromagnetic stirring and light reduction are very effective in reducing center segregation, but they require a suitable solids ratio and can easily lead to internal cracks and surface quality problems.

[0003] A search revealed a patent, CN201811014372.4, which describes a method for lightly reducing the solidification of large square billets using a combination of flat rolls and convex rolls. Based on continuous casting technology and billet forming theory, it performs model calculations on the heat transfer during solidification and the liquid phase cavity of large square billets. Under different steel grades, drawing speeds, cooling conditions, and superheat, it calculates the three-dimensional temperature field distribution, two-phase region, solid phase thickness, and solid fraction fs at each leveling machine. For the front leveling machine, which is far from the solidification end, a flat roll leveling machine is still used for compression casting of the billet. For the rear leveling machine, which is closer to the solidification end, a convex roll leveling machine is used for compression casting of the billet. The lightly reducing method uses a combination of flat roll and convex roll leveling machines to control the light reduction at the solidification end of the billet, reducing central porosity, shrinkage cavities, and segregation, thereby improving the internal quality of the rolled product. Another patent, CN201811014372.4, describes a method for controlling the dispersed segregation of large square billets used for heavy rail steel. (201910660560.2) Continuous casting electromagnetic stirring adopts secondary cooling electromagnetic stirring. The specific installation position of the secondary cooling electromagnetic stirrer is within 5.0–6.0 m from the molten steel surface of the crystallizer. The stirring current frequency is 6.0–8.0 Hz, and the magnetic field strength is 250 × 10⁻⁴–300 × 10⁻⁴ T. The superheat of the molten steel in the tundish is 30–40℃. The cooling water volume in the secondary cooling stage of continuous casting is 0.33–0.34 L / kg steel. The solidification structure of the rails exhibits a smooth transition, significantly improved uniformity of solidification structure and composition, and a significant reduction in the area of ​​dispersed segregation.

[0004] A search of patents and literature reveals that during continuous casting, superheat, electromagnetic stirring control, secondary cooling water control, and light reduction control all significantly impact the core quality of the billet. The principle of light reduction at the solidification end of continuously cast billets involves applying pressure to the billet using casting rolls to deform it. This serves two purposes: firstly, it counteracts solidification shrinkage during solidification, preventing the absorption of concentrated molten steel at the solidification end; secondly, it squeezes out the concentrated molten steel enriched in the two-phase region, allowing it to remix and dilute, effectively eliminating center segregation. During light reduction, the reduction efficiency is the efficiency with which the reduction amount is transferred to the solidification front during billet deformation. The reduction efficiency measures the degree to which the solidified shell consumes the surface area of ​​the billet during light reduction. The reduction efficiency is crucial for determining a reasonable reduction amount to compensate for solidification shrinkage at the solidification front. For small square billet castings, the secondary cooling water control is basically to spray water evenly around the perimeter, and the water volume of the secondary cooling water is consistent. This will cause the cross-section of the liquid core of the billet to be circular. In the subsequent light pressing process, the billet deformation resistance is large, the pressing effect is poor, and the center segregation fluctuation is large. Summary of the Invention

[0005] To further improve the center segregation of small square billets, this invention provides a continuous casting method for improving the center segregation of high carbon steel in small square billets. Taking 160*160mm high carbon steel small square billets as the target of improvement, different superheats, different secondary cooling water spray ratios, and appropriate light reduction processes are controlled to stably control the center segregation index of high carbon steel in small square billets at 1.05 and below.

[0006] To achieve the objectives of this invention, the technical solution adopted is as follows:

[0007] A continuous casting method for improving center segregation in high-carbon steel small billets, with billet dimensions of 160mm*160mm, includes the following steps: After refining, the high-carbon steel molten steel enters the tundish, with superheat controlled at 17-25℃, and electromagnetic stirring is employed in the crystallizer; the secondary cooling water is controlled using a strategy of separate control for the inner and outer curved surfaces and the two side surfaces, with the ratio of water content on the inner and outer curved surfaces to that on the two side surfaces controlled within a range of 2.5-3.5 (specifically, the water content on the inner and outer curved surfaces is 0.8-1.2 L / kg, and the water content on the two side surfaces is 0.3-0.42 L / kg); at the end of the billet solidification, a dynamic light reduction process is used, with reduction occurring within a solid fraction range of 0.2-0.6, and a reduction amount of 10-15mm. Using this continuous casting method, the center segregation index of the billet is reduced from 1.18 to 1.05 or below.

[0008] Preferably, the electromagnetic stirring current of the crystallizer is 250-350A and the frequency is 2.5-3HZ.

[0009] Compared with the prior art, the present invention has the following technical advantages:

[0010] By controlling the superheat of the tundish and the electromagnetic stirring process of the crystallizer, a high equiaxed crystal ratio of small square billets is ensured. On this basis, the secondary cooling water regulation process is used to control the cross-section of the molten steel at the end of the solidification of the continuous casting billet to a specific morphology, so as to match it with the light reduction process. In the end, the center segregation of the continuous casting billet is effectively improved and the segregation index is reduced to 1.05 and below. Attached Figure Description

[0011] Figure 1 The pressing efficiency of the pressure roller at different solidification periods when the secondary cooling water ratio is 1.

[0012] Figure 2 The pressing efficiency of the pressure roller at different solidification periods when the secondary cooling water ratio is 2.

[0013] Figure 3 The pressing efficiency of the pressure roller at different solidification periods when the secondary cooling water ratio is 3.

[0014] Figure 4 Comparison of center segregation index of LX82B steel billet in Example 1;

[0015] Figure 5 Comparison of center segregation index of LX82B steel billet in Example 2;

[0016] Figure 6 Comparison of center segregation index of GCr15 steel billet in Example 3.

[0017] Figure 7 Comparison of center segregation index of GCr15 steel billet in Example 4. Detailed Implementation

[0018] This invention takes LX82B and GCr15 steel produced from 160mm*160mm small square billets as the research object, selecting a 300mm section (1 / 4 of the cross-section) before and after the light reduction position as the research object. A finite element analysis model of billet deformation during single-roll reduction in a casting machine was established using MSC.MARC finite element software, and the deformation behavior of the billet during the continuous casting process of small square billets was quantitatively analyzed. During the casting process, due to the inconsistent water ratio of the secondary cooling water, there are differences in the thickness of the inner and outer arc liquid cores and the thickness of the left and right arc liquid cores. P is defined as the water ratio of the inner and outer arc surfaces / the water ratio of the two sides. When P is 1, it represents the conventional secondary cooling water arrangement for small square billets, and the relationship between the reduction amount and reduction efficiency is as follows: Figure 1 As shown. From Figure 1 As can be seen, when pressing is performed at different positions at the end of the solidification of the billet (fs is the solid fraction at the center of the billet), the pressing efficiency decreases as the solid fraction at the center increases. Figure 2 and Figure 3The graph shows the relationship between reduction amount and reduction efficiency when P=2 and P=3. As can be seen from the graph, at the same reduction position, the single-roll reduction efficiency is significantly improved. This is mainly because changing the distribution ratio of the secondary cooling water in the continuously cast billet results in a noticeable change in the liquid core of the billet. The thickness of the inner and outer arc liquid core decreases, while the thickness of the liquid core on both sides increases, leading to a significant improvement in reduction efficiency at the same reduction position.

[0019] The present invention will be further described in detail below with reference to the embodiments:

[0020] Example 1:

[0021] The present invention employs the following specific process steps for casting LX82B steel on a 160mm*160mm continuous casting machine.

[0022] After refining, LX82B steel enters the tundish with a superheat control of 20°C. Electromagnetic stirring is used in the crystallizer with a stirring current of 350A and a frequency of 3Hz. Secondary cooling water control employs a strategy of separate control for the inner and outer curved surfaces and the two side surfaces. The water content for the inner and outer curved surfaces is 1.0L / kg, and the water content for the two side surfaces is 0.33L / kg, with a control range of 3 for the ratio of inner / outer curved surface water content to two side surface water content. At the end of the billet solidification process, dynamic light reduction is used, with reduction occurring within a solid fraction range of 0.2-0.6. Four consecutive reduction rollers are used, with single-roller reductions of (2.5mm, 3.5mm, 3mm, 3mm) and a reduction amount of 12mm. After casting, three billets are randomly selected, and the carbon content at the center of the billet is analyzed to obtain the billet segregation index (C / C0). The billet center segregation index is compared with that of conventional processes. Figure 4 As shown in the figure, inventions 1-3 respectively represent the segregation values ​​obtained from three casting billets.

[0023] Example 2:

[0024] This embodiment involves casting LX82B steel on a 160mm*160mm continuous casting machine, primarily based on the optimal process steps under different reduction control conditions in Embodiment 1, as detailed below:

[0025] After refining, LX82B steel enters the tundish with a superheat control of 22℃. Electromagnetic stirring is used in the crystallizer with a stirring current of 350A and a frequency of 3Hz. Secondary cooling water control employs a strategy of separate control for the inner and outer curved surfaces and the two side surfaces. The water content for the inner and outer curved surfaces is 0.8L / kg, and for the two side surfaces it is 0.25L / kg, with a control range of 3.2 for the ratio of inner / outer curved surface water content to two side surface water content. At the end of the billet solidification process, dynamic light reduction is used, with reduction occurring within a solid fraction range of 0.2-0.6. Four consecutive reduction rollers are used, with single-roller reductions of (2.5mm, 4mm, 3.5mm, 3mm), for a total reduction of 13mm. After casting, three billets are randomly selected, and the carbon content at the center of the billet is analyzed to obtain the billet segregation index (C / C0). The billet center segregation index is compared with that of conventional processes. Figure 4 As shown in the figure, inventions 1-3 represent the segregation values ​​of three casting billets, respectively.

[0026] Example 3

[0027] The present invention employs the following specific process steps for casting GCr15 steel on a 160mm*160mm continuous casting machine. After refining, the molten GCr15 steel enters the tundish with a superheat control of 18℃. Electromagnetic stirring is used in the crystallizer with a stirring current of 300A and a frequency of 3.5Hz. The secondary cooling water is controlled separately for the inner and outer curved surfaces and the two side surfaces, with a water content of 1.2L / kg for the inner and outer curved surfaces and 0.4L / kg for the two side surfaces. The ratio of water content for the inner and outer curved surfaces to that for the two side surfaces is controlled within a range of 3. At the end of the billet solidification, a dynamic light reduction process is used, with reduction occurring within a solid fraction range of 0.2-0.6. Four consecutive reduction rollers are used, with a single-roll reduction distribution of (2.5mm, 4.5mm, 4.5mm, 3.5mm), and a reduction amount of 15mm. After casting, three billets were randomly selected and the carbon content at the center of the billets was analyzed to obtain the billet segregation index (C / C0). The billet center segregation index was compared with that of conventional processes. Figure 5 As shown in the figure, inventions 1-3 represent the segregation values ​​of three casting billets, respectively.

[0028] Example 4

[0029] This embodiment involves casting GCr15 steel on a 160mm*160mm continuous casting machine, mainly based on the optimal process steps under the pressure control of different embodiments 3, as follows:

[0030] After refining, the GCr15 steel molten steel enters the tundish with a superheat control of 25℃. Electromagnetic stirring is used in the crystallizer with a stirring current of 350A and a frequency of 3.5Hz. The secondary cooling water is controlled separately for the inner and outer curved surfaces and the two side surfaces. The water content for the inner and outer curved surfaces is 1.4L / kg, and for the two side surfaces it is 0.45L / kg, with a ratio of 3.1. At the end of the billet solidification, a dynamic light reduction process is used, with reduction occurring within a solids fraction range of 0.2-0.6. Four consecutive reduction rollers are used, with single-roller reductions of (2.5mm, 4mm, 4mm, 2.5mm) and a reduction amount of 13mm. After casting, three billets are randomly selected, and the carbon content at the center of the billet is analyzed to obtain the billet segregation index (C / C0). The billet center segregation index is compared with that of conventional processes. Figure 5 As shown in the figure, inventions 1-3 represent the segregation values ​​of three casting billets, respectively.

[0031] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A continuous casting method for improving center segregation in high-carbon steel small square billets, characterized in that: The small square billet has dimensions of 160mm*160mm and includes the following steps: After refining, high-carbon steel molten steel enters the tundish, with superheat controlled at 17-25℃, and electromagnetic stirring is used in the crystallizer; the secondary cooling water is controlled using a strategy of separate control of the inner and outer arc surfaces and the two sides, with the ratio of water content of the inner and outer arc surfaces to the water content of the two sides controlled within a range of 3-3.5; at the end of the solidification of the small square billet casting, a dynamic light reduction process is used, with reduction occurring within a solid fraction range of 0.2-0.6, using four consecutive reduction rollers, with a reduction amount of 10-15mm. The high-carbon steel is LX82B steel; the electromagnetic stirring current in the crystallizer is 250-350A, and the frequency is 2.5-3HZ; the water content of the inner and outer arc surfaces is 0.8-1.2L / kg, and the water content of the two sides is 0.3-0.42L / kg.

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