A continuous casting method for improving the central porosity of small billet low-carbon steel

Abstract

The present invention belongs to the technical field of continuous casting of steel, and discloses a continuous casting method for improving the central porosity of low-carbon steel in small billets. The molten low-carbon steel enters the tundish with a superheat degree controlled at 17 - 35°C after refining; electromagnetic stirring is adopted in the mold, with a stirring current of 250 - 350 A and a frequency of 2.5 - 3.5 HZ; the secondary cooling water control adopts separate control for the inner and outer arc surfaces and the two side surfaces, the specific water volume of the inner and outer arc surfaces is 0.8 - 1.2 L / kg, the specific water volume of the two side surfaces is 0.3 - 0.42 L / kg, and the control range of the specific water volume ratio of the inner and outer arc surfaces to the two side surfaces is 2 - 4; the dynamic soft reduction process is adopted at the solidification end of the billet, and the reduction is carried out within the range of a solid fraction of 0.5 - 0.8, with a reduction amount of 10 - 15 mm, and the single-roll heavy reduction is carried out within the range of a solid fraction of 0.9 - 1.0, with a reduction amount of 8 - 12 mm. When producing low-carbon steel according to this continuous casting method, the central shrinkage cavity of the billet disappears.

Description

Technical Field

[0001] The present invention belongs to the technical field of continuous casting of steel, and in particular, to a continuous casting method for improving the central porosity of low-carbon steel in small billets, belonging to the metallurgical technology field. Background Art

[0002] During the solidification process of molten steel, as the temperature decreases, columnar crystals in the billet are well-developed, which easily causes phenomena such as bridging and defects such as central shrinkage cavities. The central shrinkage cavity in small billets is not easily rolled up during the subsequent rolling process, and product defects such as cracking will occur, affecting the properties of steel.

[0003] The Chinese patent document (application number: 201210204232.X) discloses a continuous casting method for improving the central shrinkage cavity of high-carbon steel small billets. By controlling the superheat degree, the argon blowing amount of the submerged nozzle, full protection casting, adjusting the secondary cooling water ratio and casting speed, mold electromagnetic stirring and soft reduction at the solidification end, the specific process parameters are as follows: full protection casting is adopted, the argon blowing amount of the submerged nozzle is controlled at 15 - 20 L / min, the average casting speed corresponding to each strand of the mold is 2.6 - 3.0 m / min, the cooling water ratio of the secondary cooling water in the secondary cooling section of each strand of the mold is 0.8 - 1.2 L / kg, and mist cooling is used; full low superheat casting is adopted, the superheat degree is 20 - 40 °C, mold electromagnetic stirring under the conditions of 460 - 480 A and 5 - 7 Hz and soft reduction at the solidification end of 0.4 - 4.5 m are adopted. The Chinese patent document (application number: 201210584888.9) discloses a process for improving the solidification quality of high-carbon steel billets, which mainly includes the following process flows: 1. 100t converter; 2. 100t-LF refining furnace; 3. Continuous casting of 150mm×150mm small billets; 4. Heating furnace; 5. Rolling wire rods by a high-speed wire rod rolling mill. By optimizing process parameters such as the secondary cooling water ratio, the distribution of cooling water in each cooling section in the secondary cooling zone, the superheat degree of molten steel, and the continuous casting speed, the internal quality of the continuous casting billet is improved. Combining the above existing technologies, it can be seen that during the continuous casting process of molten steel, the superheat degree, electromagnetic stirring control, secondary cooling water control, and soft reduction control will all have a greater impact on the quality of the billet core. However, in the existing technologies, there is no method of separately controlling the inner and outer arc surfaces and the side surfaces of the secondary cooling water to improve the reduction efficiency and the two-zone and post-solidification separate control methods of continuous casting soft reduction. The principle of soft reduction at the solidification end of the continuous casting billet is to apply a certain pressure to the billet through casting rolls at the solidification end of the billet to deform the billet. On the one hand, reduction is carried out in the two-phase zone to offset the solidification shrinkage during the solidification process of the billet and reduce the central shrinkage cavity; on the other hand, reduction is carried out after solidification to effectively eliminate the central shrinkage cavity. During the implementation of soft reduction, the reduction efficiency is the efficiency of the reduction amount transferred to the solidification front of the billet when the billet is reduced and deformed, and the reduction efficiency is used to measure the consumption degree of the solidified shell on the surface of the billet during soft reduction. The reduction efficiency is crucial for formulating a reasonable reduction amount to supplement the solidification shrinkage at the solidification front. For small billets, the secondary cooling water control is basically uniform spraying around, and the secondary cooling water ratio is the same, which will cause the cross-section of the liquid core of the billet to be circular. In the subsequent soft reduction process, the billet has a large deformation resistance, the reduction effect is poor, and it is difficult to control the central shrinkage cavity. Summary of the Invention

[0004] The object of the present invention is to improve the continuous casting method for the central porosity of low-carbon steel small billets, which can eliminate the central shrinkage cavity of low-carbon steel small billets.

[0005] In order to achieve the object of the present invention, the technical solution adopted is as follows:

[0006] A continuous casting method for improving the central porosity of low-carbon steel billets includes the following steps:

[0007] After being refined, the low-carbon steel molten steel enters the tundish, and the superheat is controlled at 17 - 35°C (preferably 17 - 25°C); electromagnetic stirring of the mold is adopted, the stirring current is 250 - 350 A, and the frequency is 2.5 - 3.5 HZ (preferably 2.5 - 3 HZ); the secondary cooling water control adopts a strategy of separate control for the inner and outer arc surfaces and both side surfaces. The specific water ratio for the inner and outer arc surfaces is 0.8 - 1.2 L / kg, and the specific water ratio for both side surfaces is 0.3 - 0.42 L / kg. The control range of the specific water ratio of the inner and outer arc surfaces / the specific water ratio of both side surfaces is 2 - 4 (preferably 2.5 - 3.5); the dynamic soft reduction process is adopted at the solidification end of the billet. The reduction is carried out within the range of solid fraction of 0.5 - 0.8, and the reduction amount is 10 - 15 mm. The single-roll heavy reduction is carried out within the range of solid fraction of 0.9 - 1.0, and the reduction amount is 8 - 12 mm. By producing low-carbon steel according to this continuous casting method, the central shrinkage cavity of the billet disappears.

[0008] Furthermore, after being refined, the low-carbon steel molten steel enters the tundish, and the superheat is controlled at 17 - 25°C, cooperating with the electromagnetic stirring of the mold. At the same time, the stirring current is 250 - 350 A, and the frequency is 2.5 - 3 HZ, which can effectively reduce the columnar crystal ratio of the small billet.

[0009] The secondary cooling water control method of the present invention can effectively control the cross-sectional morphology of the molten steel at the center of the solidification end of the continuous casting billet, thereby effectively improving the reduction efficiency and the quality of the core part of the billet.

[0010] Based on the above process, the present invention cooperates with the dynamic soft reduction process at the solidification end of the billet. Specifically, the reduction is carried out within the range of solid fraction of 0.5 - 0.8, and the reduction amount is 10 - 15 mm. At the same time, the single-roll heavy reduction with a reduction amount of 8 - 12 mm is carried out within the range of solid fraction of 0.9 - 1.0, which can improve the central shrinkage cavity of the continuous casting billet until the central shrinkage cavity of the billet disappears.

[0011] Compared with the prior art, the beneficial effects obtained by the present invention are as follows: The method of the present invention can effectively improve the reduction efficiency of low-carbon steel small billets, improve the quality of the core part of the billets, and the central shrinkage cavity disappears. Description of the Drawings

[0012] Figure 1 For the reduction efficiency of the pressure roller at different solidification periods when the ratio of the secondary cooling water specific flow rate is 1;

[0013] Figure 2 For the reduction efficiency of the pressure roller at different solidification periods when the ratio of the secondary cooling water specific flow rate is 2;

[0014] Figure 3 For the reduction efficiency of the pressure roller at different solidification periods when the ratio of the secondary cooling water specific flow rate is 3;

[0015] Figure 4 For comparison of the central shrinkage cavity of Q195 steel billets;

[0016] Figure 5 For comparison of the central shrinkage cavity of 35K steel billets;

[0017] Figure 6 For the morphology of the unfrozen molten steel at the center of the solidification end of the present invention. Detailed implementation manners

[0018] First, taking Q195 and 35K steels produced by 160mm*160mm small square billets as the research objects, selecting the billets with a length of 300mm and a 1 / 4 cross-section before and after the soft reduction position as the research objects, a finite element analysis model of the billet deformation during single-roll reduction of the continuous 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 of the billet, due to the inconsistency of the secondary cooling water flow rate per unit mass, there are differences in the liquid core thickness of the inner and outer arcs and the left and right arcs of the billet. Define P as the ratio of the secondary cooling water flow rate of the inner and outer arc surfaces to the secondary cooling water flow rate of the two side surfaces. When P is 1, it is the conventional secondary cooling water arrangement for small square billets, and the relationship between the reduction amount and the reduction efficiency is as Figure 1 shown. From Figure 1 it can be seen that when reduction is carried out at different positions at the solidification end of the billet (fs is the solid fraction at the center of the billet), the reduction efficiency decreases with the increase of the solid fraction at the center. Figure 2 And Figure 3 are the relationship diagrams of the reduction amount and the reduction efficiency when P = 2 and P = 3. It can be seen from the figure that at the same reduction position, the single-roll reduction efficiency has a large increase. This is mainly because after changing the distribution ratio of the secondary cooling water of the continuous casting billet, there are obvious changes in the liquid core at the center of the billet. The liquid core thickness of the inner and outer arcs decreases, and the liquid core thickness of the two sides increases. At the same reduction position, the reduction efficiency has an obvious increase. The most core point of the present invention is the combined control of the secondary cooling water and the soft reduction. The previous superheat control and electromagnetic stirring belong to the normal process, but they are also essential parameters in the production process. This part is the correlation between the secondary cooling water control and the soft reduction control obtained through a large number of experiments and tests. It determines the selection of the secondary cooling water control parameters and the soft reduction control parameters adopted by the present invention.

[0019] The improvement of the reduction efficiency during the soft reduction of the billet corresponds to an obvious improvement effect on the central shrinkage cavity. The present invention has been widely applied in actual production.

[0020] The soft reduction of the present invention adopts a hierarchical reduction strategy. In the two-phase region (when the solid fraction fs is 0.5 - 0.8), the multi-roll reduction amount is controlled to be 10 - 15mm to supplement the solidification shrinkage size; after solidification (when the solid fraction fs is 0.9 - 1), the single-roll large reduction amount is 8 - 10mm to completely eliminate the central shrinkage cavity defect.

[0021] The present invention will be further described in detail below in conjunction with specific embodiments:

[0022] Embodiment 1:

[0023] The following specific process steps are adopted for pouring Q195 steel on a 160mm*160mm continuous caster in the present invention.

[0024] The molten steel of Q195 steel enters the tundish after refining, and the superheat is controlled at 23°C; electromagnetic stirring in the mold is adopted, the stirring current is 350A, and the frequency is 3.5HZ; the secondary cooling water control adopts a strategy of separate control for the inner and outer arc surfaces and the two side surfaces. The specific water ratio for the inner and outer arc surfaces is 1.1L / kg, and the specific water ratio for the two side surfaces is 0.3L / kg. The control range of the specific water ratio of the inner and outer arc surfaces to the two side surfaces is 3.67; the dynamic soft reduction process is adopted at the solidification end of the slab. Multi-roll reduction is carried out within the range of solid fraction of 0.5 - 0.8, and the reduction amount is 13mm. Single-roll heavy reduction is carried out within the range of solid fraction of 0.9 - 1.0, and the reduction amount is 11mm. By using this continuous casting method to produce low-carbon steel, the central shrinkage cavity of the slab disappears. After pouring, 4 slabs are randomly taken, longitudinally cut along the center line, polished and corroded. The center tissues before and after optimization are as Figure 4 shown.

[0025] Embodiment 2

[0026] The following specific process steps are adopted for pouring 35K steel on a 160mm*160mm continuous caster in the present invention.

[0027] The molten steel of 35K steel enters the tundish after refining, and the superheat is controlled at 21°C; electromagnetic stirring in the mold is adopted, the stirring current is 300A, and the frequency is 3.5HZ; the secondary cooling water control adopts a strategy of separate control for the inner and outer arc surfaces and the two side surfaces. The specific water ratio for the inner and outer arc surfaces is 0.9L / kg, and the specific water ratio for the two side surfaces is 0.4L / kg. The control range of the specific water ratio of the inner and outer arc surfaces to the two side surfaces is 2.25; the dynamic soft reduction process is adopted at the solidification end of the slab. Multi-roll reduction is carried out within the range of solid fraction of 0.5 - 0.8, and the reduction amount is 15mm. Single-roll heavy reduction is carried out within the range of solid fraction of 0.9 - 1.0, and the reduction amount is 8mm. By using this continuous casting method to produce low-carbon steel, the central shrinkage cavity of the slab disappears. After pouring, 4 slabs are randomly taken, longitudinally cut along the center line, polished and corroded. The center tissues before and after optimization are as Figure 5 shown.

[0028] For the secondary cooling water control method of the present invention, the water spraying on the upper and lower surfaces is stronger, and the solidification of the molten steel is faster. The water spraying on the two side surfaces is weaker, and the solidification of the molten steel is slower. In this way, the molten steel that is not solidified at the center of the solidification end is oval in cross-section (see Figure 6 ). The oval liquid core is easier to be pressed down during the soft reduction process, improving defects such as molten steel segregation and shrinkage cavity.

[0029] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and concept of the present invention, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

Claims

1. A continuous casting method for improving the central porosity of low-carbon steel in small billets, characterized in that: It includes the following steps: The low-carbon steel molten steel enters the tundish after refining, and the superheat is controlled at 17 - 35°C; the mold electromagnetic stirring is adopted, the stirring current is 250 - 350 A, and the frequency is 2.5 - 3.5 HZ; the secondary cooling water control adopts the strategy of separate control of the inner and outer arc surfaces and the two side surfaces. The specific water volume of the inner and outer arc surfaces is 0.8 - 1.2 L / kg, the specific water volume of the two side surfaces is 0.3 - 0.42 L / kg, and the control range of the specific water volume ratio of the inner and outer arc surfaces to the two side surfaces is 2 - 4; the dynamic soft reduction process is adopted at the solidification end of the slab, the reduction is carried out within the solid fraction range of 0.5 - 0.8, the reduction amount is 10 - 15 mm, and the single-roll heavy reduction is carried out within the solid fraction range of 0.9 - 1.0, and the reduction amount is 8 - 12 mm.

2. The continuous casting method for improving the central porosity of small billet low-carbon steel according to claim 1, characterized in that: The superheat of the tundish is controlled at 17 - 25°C.

3. The continuous casting method for improving the central porosity of small billet low-carbon steel according to claim 1, characterized in that: The frequency of the mold electromagnetic stirring is 2.5 - 3 HZ.

4. The continuous casting method for improving the central porosity of small billet low-carbon steel according to claim 1, characterized in that: The control range of the specific water volume ratio of the inner and outer arc surfaces to the two side surfaces is 2.5 - 3.5.

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