Method and device for improving segregation of low alloy steel cast slab

Abstract

The application relates to a method and device for improving the segregation of low-alloy steel cast slabs, which comprises feeding a mixture of pure iron particles and protective slag into the liquid surface of the steel liquid in a crystallizer during continuous casting, wherein the pure iron particles contain nano-powder, and the nano-powder is one or both of TiN and TiC; and feeding protective slag into other areas of the continuous casting slab. By adding the pure iron particles containing nano-powder near the thickness center of the continuous casting slab during continuous casting, the temperature difference in the thickness direction of the continuous casting slab is reduced, the superheat of the center of the continuous casting slab is reduced during the formation of the slab shell, equiaxed crystals are formed, the formation of developed columnar crystals is inhibited, and the composition segregation in the continuous casting slab is reduced.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking and continuous casting technology, and specifically relates to a method and apparatus for improving segregation in low alloy steel billets. Background Technology

[0002] Continuous casting is a crucial process in modern metallurgical production. During solidification, central segregation often occurs in continuously cast billets, causing the formation of brittle structures such as martensite or bainite in the center during rolling, thus deteriorating the steel's mechanical properties. Furthermore, central segregation is often accompanied by central porosity and central cracks, further reducing the internal density and mechanical properties of the billet. Low-alloy high-strength steel, produced by adding small amounts of alloying elements to improve its properties, is widely used in engineering structural steel. In the production of low-alloy high-strength steel, central segregation in continuously cast billets is particularly severe, resulting in a significantly lower yield rate compared to ordinary carbon steel.

[0003] Chinese patent application (application number 202110420992.3) discloses a method for reducing center segregation in low-alloy steel continuously cast billets, characterized by addressing the problem of low internal segregation pass rate in low-alloy high-strength steel billets. Firstly, based on the production data of the target continuous casting machine, the carbon equivalent range of the target steel grade continuously cast billet with acceptable center segregation is analyzed. The composition content of the target steel grade continuously cast billet is then calculated backwards to optimize the optimal composition content range of molten steel and the optimal casting parameter range corresponding to the carbon equivalent of the target steel grade during production. Therefore, based on the aforementioned optimal composition content range and optimal casting parameter range, low-alloy steel grade continuously cast billets with acceptable center segregation can be produced. Optimizing the molten steel composition and casting parameters based on the carbon equivalent range allows for more accurate identification of the production conditions for preparing target steel grade continuously cast billets with acceptable internal segregation quality, thereby effectively improving the center segregation of low-alloy steel continuously cast billets and enhancing the internal segregation quality of low-alloy steel continuously cast billets. The patent document does not address the issue of selective crystallization during the solidification process of continuously cast billets, and therefore cannot fundamentally solve the problem of central segregation in continuously cast billets and the series of quality problems caused by it.

[0004] Chinese patent application (application number 202311517739.5) discloses a method for controlling reduction in continuous casting to improve macroscopic and semi-macroscopic segregation in high-carbon low-alloy steel. This method, belonging to the field of continuous casting technology, is characterized by providing a method for controlling reduction in continuous casting to improve macroscopic and semi-macroscopic segregation in high-carbon low-alloy steel. The high-carbon low-alloy steel has an alloy content of 1.5-2.5% by mass, a carbon content of 0.50-0.65% by mass, and a Mn content of 0.5-1.0% by mass. Within the continuous casting reduction region, the reduction is controlled by the change in the solid fraction at the center of the billet. The solid fraction at the center of the reduction region is 0.09-1.0, and the total reduction is 10 mm. Within the continuous casting reduction region corresponding to a solid fraction at the center of less than 0.4, the total reduction is not less than 1 mm and not more than 2 mm. This invention, by controlling the reduction process, simultaneously suppresses the formation of macroscopic segregation and semi-macroscopic point segregation in the equiaxed crystal region. This patent document addresses the problem of segregation during the solidification process of continuously cast billets by implementing a pressing process at the end of solidification to improve the segregation phenomenon in the billets. However, during continuous casting, parameters such as steel composition, superheat, casting speed, and cooling cannot be accurately controlled, making it impossible to accurately locate the pressing position at the end. Therefore, it cannot fundamentally solve the problem of center segregation in the continuously cast billets and the series of quality problems caused by it.

[0005] In summary, existing technologies for producing low-alloy steel billets are prone to compositional segregation during continuous casting, resulting in uneven microstructure and properties. This can lead to defects such as cracking during processing, severely affecting product quality. Summary of the Invention

[0006] To address the aforementioned segregation phenomenon in the continuous casting of low-alloy steel, which leads to uneven microstructure and delamination cracking during processing, thus affecting product quality, there is an urgent need to develop methods and devices to improve the segregation phenomenon in the continuous casting of low-alloy steel. This would significantly improve product quality and prevent cracking caused by microstructure differences due to compositional segregation.

[0007] To achieve the above objectives, the present invention employs the following technical solution: A method for improving segregation in low-alloy steel billets involves adding material to the surface of molten steel in the crystallizer during continuous casting: adding a mixture of pure iron particles and protective slag in the central region of the billet's thickness, wherein the pure iron particles contain nanoparticles, the nanoparticles being one or both of TiN and TiC; and adding protective slag in other regions of the billet.

[0008] To prevent the slag added to the crystallizer from absorbing moisture, the sealed packaging of the slag should be opened 30 minutes before continuous casting. The feeding process should be carried out 30 minutes before the start of continuous casting. The superheat of the molten steel should be controlled at 18-22℃, and the billet pulling speed should be 0.9-1.1m / min.

[0009] In the mixture of pure iron particles and protective slag, the pure iron particles and protective slag are mixed at a weight percentage of (6-8):1.

[0010] The thickness of the continuously cast billet is 200-300 mm, the width is 1000-2200 mm, and the thickness center area refers to the area 10-20 mm above and below the thickness center line.

[0011] The feeding height is 400-600mm from the molten steel surface in the crystallizer; the feeding speed is 0.5-0.7kg / t in the center of the continuous casting billet thickness area and 0.40-0.48kg / t in other areas.

[0012] The preparation process of the pure iron particles is as follows: smelting is carried out in a vacuum induction furnace. The raw material is industrial pure iron, and one or both of the following nano-powders, TiN or TiC, with a particle size of 0.3-0.6 μm, are added. The amount of nano-powder added accounts for 1wt%-2wt% of the molten steel. Before melting begins, a vacuum operation is performed to bring the vacuum degree of the equipment to below 10 Pa. Then, electric melting is carried out. After melting, the temperature of the melt is raised to 1636-1736℃, and atomization granulation is performed. The atomization nozzle has an orifice diameter of 4-6 mm, the blowing gas is nitrogen, the blowing gas temperature is -30-40℃, the blowing gas pressure is 0.2-0.4 MPa, and the atomization speed of the molten steel is 6-10 kg / min. After the molten steel is broken once by the blowing gas, it cools and solidifies during the fall in the atomization chamber to obtain pure iron particles containing nano-powder. The pure iron particles are collected and sieved, and pure iron particles with a particle size of 1-4 mm are selected and sealed for storage.

[0013] The chemical composition of the industrial pure iron, by weight percentage, is as follows: C: 0.005%–0.010%, Si: 0.008%–0.015%, Mn: 0.01%–0.03%, P: 0.002%–0.004%, S: 0.001%–0.003%, with the remainder being Fe and unavoidable impurities.

[0014] An apparatus for improving segregation in low-alloy steel billets includes a crystallizer, a No. 1 slag feeder, and a No. 2 slag feeder. The No. 1 and No. 2 slag feeders are fixed together side by side by welding the outer shell and are driven by a motor to move horizontally back and forth above the crystallizer. The No. 2 slag feeder feeds material above the center region of the thickness of the continuously cast billet, while the No. 1 slag feeder feeds material to other parts of the liquid surface in the crystallizer.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. The present invention provides a method for improving segregation in low-alloy steel billets. By adding pure iron particles containing nanopowder near the center of the billet thickness during continuous casting, the temperature difference in the thickness direction of the continuously cast billet is reduced. This reduces the overheating at the center of the billet during the formation of the billet shell, while simultaneously forming equiaxed crystals, inhibiting the formation of well-developed columnar crystals, and alleviating compositional segregation within the billet.

[0016] 2. The present invention improves the segregation of low alloy steel billets by adding nanoparticles during the preparation of pure iron particles, followed by atomization granulation, so that the nanoparticles are evenly distributed in the pure iron particles. The nanoparticles are then added to the molten steel during continuous casting, playing a good nucleation role.

[0017] 3. The device for improving segregation in low-alloy steel billets of this invention involves adding a mixture of pure iron particles and protective slag near the center of the billet thickness, while adding only protective slag to other parts. This reduces the temperature difference along the thickness direction of the billet. The carbon segregation index of this invention is controlled at 0.98-1.04, the manganese segregation index at 0.98-1.02, the niobium segregation index at 0.96-1.05, and the titanium segregation index at 0.96-1.03. In contrast, the carbon segregation index in the comparative example is 0.86-1.17, the manganese segregation index is 0.91-1.07, the niobium segregation index is controlled at 0.86-1.14, and the titanium segregation index is controlled at 0.90-1.08. This significantly improves the degree of segregation, increases the efficiency of the heating furnace and subsequent rolling mill, and reduces production costs.

[0018] 4. The device used in this invention to improve the segregation of low alloy steel billets has a simple structure, low investment, and does not affect the normal operation of other equipment. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of the thickness center region of the continuous casting pure iron particle addition device of the present invention during the feeding process.

[0020] Figure 2 This is a side cross-sectional view of the continuous casting device for adding pure iron particles according to the present invention.

[0021] In the diagram: 1. Immersion nozzle; 2. No. 1 slag feeder; 3. No. 2 slag feeder; 4. Crystallizer; 5. Protective slag; 6. Pure iron particles; 7. Continuous casting billet shell; 8. Molten steel; 9. Nucleating agent; 10. Equiaxed crystals; 11. Motor. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0023] To further describe the present invention, the following detailed description is provided in conjunction with embodiments: like Figure 1 As shown, a method for improving segregation in low-alloy steel billets involves adding material to the surface of molten steel in the crystallizer 4 during continuous casting: adding a mixture of pure iron particles 6 and protective slag 5 to the thickness center region of the continuously cast billet, wherein the pure iron particles 6 contain nanoparticles, the nanoparticles being one or both of TiN and TiC; and adding protective slag 5 to other regions of the continuously cast billet.

[0024] To prevent the slag added to the crystallizer from absorbing moisture, the sealed packaging of the slag should be opened 30 minutes before continuous casting. The feeding process should be carried out within 30 minutes before the start of continuous casting. The superheat of the molten steel should be controlled at 18-22℃, and the billet pulling speed should be 0.9-1.1m / min.

[0025] In the mixture of pure iron particles 6 and protective slag 5, the pure iron particles 6 and protective slag 5 are mixed in a weight percentage of (6-8):1.

[0026] The thickness of the continuously cast billet is 200-300 mm, and the width is 1000-2200 mm. The thickness center area refers to the region 10-20 mm above and below the thickness center line. The feeding height is 400-600 mm from the molten steel surface of the crystallizer. The feeding rates are as follows: 0.5-0.7 kg / t for the thickness center area of ​​the continuously cast billet (2# slag feeder 3), and 0.40-0.48 kg / t for other areas (1# slag feeder 2).

[0027] The preparation process of the pure iron particles 6 is as follows: smelting is carried out in a vacuum induction furnace. The raw material is industrial pure iron, and one or both of the following nano-powders, TiN or TiC, with a particle size of 0.3-0.6 μm, are added. The amount of nano-powder added accounts for 1wt%-2wt% of the molten steel. Before melting begins, a vacuum operation is performed to bring the vacuum degree of the equipment to below 10 Pa. Then, electric melting is carried out. After melting, the temperature of the melt is raised to 1636-1736℃, and atomization granulation is performed. The atomization nozzle has an orifice diameter of 4-6 mm, the blowing gas is nitrogen, the blowing gas temperature is -30-40℃, the blowing gas pressure is 0.2-0.4 MPa, and the atomization speed of the molten steel is 6-10 kg / min. After the molten steel is broken once by the blowing gas, it cools and solidifies during the falling process in the atomization chamber to obtain pure iron particles containing nano-powder. The pure iron particles are collected and sieved, and pure iron particles with a particle size of 1-4 mm are selected and sealed for storage.

[0028] The chemical composition of the industrial pure iron, by weight percentage, is as follows: C: 0.005%–0.010%, Si: 0.008%–0.015%, Mn: 0.01%–0.03%, P: 0.002%–0.004%, S: 0.001%–0.003%, with the remainder being Fe and unavoidable impurities.

[0029] An apparatus used in a method for improving segregation in low-alloy steel billets includes a crystallizer 4, a slag feeder 2 (1#) and a slag feeder 3 (2#). The slag feeder 2 (1#) and the slag feeder 3 (2#) are fixed together side by side by a welded outer shell and driven by a motor 11 to move horizontally reciprocally above the crystallizer 4. The slag feeder 3 (2#) feeds material above the center region of the thickness of the continuously cast billet, while the slag feeder 2 (1#) feeds material to other parts of the liquid surface in the crystallizer 4.

[0030] Slag feeders #1 (2) and #2 (3) are arranged side-by-side by welding their outer shells together. During continuous casting, driven by motor 11, slag feeders #1 and #2 (3) move horizontally back and forth above the crystallizer 4. The slag is automatically added via a screw conveyor. Slag feeder #2 (3) adds slag to the center of the continuously cast billet, while slag feeder #1 (2) adds slag to other parts of the molten steel surface in the crystallizer 4. The outlets of the two slag feeders are located above the continuous casting crystallizer 4. Due to the significant difference in specific gravity between the pure iron particles 6 and the protective slag 5, the pure iron particles 6 containing nanopowder added by slag feeder #2 (3) pass directly through the interface of the protective slag 5 under gravity and enter the molten steel 8. The protective slag 5 added at the same time remains on top of the protective slag 5, providing insulation and anti-oxidation protection for both the molten steel 8 and the pure iron particles 6. After the pure iron particles 6 enter the molten steel 8, they absorb the heat of the molten steel 8 and gradually melt into the molten steel 8, reducing the superheat of the molten steel 8. The nano-powder TiN or TiC in the pure iron particles 6 acts as a nucleating agent 9 in the molten steel 8, playing a heterogeneous nucleation role and promoting the formation of equiaxed crystals 10 inside the molten steel.

[0031] To control segregation within the low-alloy steel continuously cast billet, the following measures are adopted in the continuous casting production process: the thickness of the continuously cast billet is 250 mm, and the width is 1000-2200 mm. Slag is added simultaneously on both sides of the continuous casting crystallizer 4. The outlet of the slag adder is located above the continuous casting crystallizer, 400-600 mm away from the steel liquid surface of the crystallizer 4. The superheat of the continuously cast steel is controlled at 18-22℃. The added pure iron particles are formed by atomization granulation, and their composition is industrial pure iron containing one or both of nano-powder TiN or TiC. Within 30 minutes before continuous casting begins, pure iron particles and protective slag are mixed at a weight percentage of 6-8:1. The mixture of pure iron particles (6) and protective slag (5) is added by slag feeder #2 (3) near the center of the continuously cast billet thickness. Further away from the center, only protective slag (5) is added by slag feeder #1 (2). During continuous casting, the slag feeders use a screw conveyor system. The feeding rate of slag feeder #1 (2) is 0.40-0.48 kg / t, and the feeding rate of slag feeder #2 (3) is 0.5-0.7 kg / t. The pure iron particles (6) added near the center of the continuously cast billet thickness absorb heat from the molten steel (8) and melt, thus lowering the temperature of the molten steel in the central region. Simultaneously, they release nanoparticles, which act as nucleating agents. During the solidification of molten steel 8, due to selective crystallization, solute elements will focus on the area where the continuous casting billet is finally solidified. By adding pure iron particles 6, a certain dilution effect is achieved after melting, which reduces the enrichment of solute elements and makes the composition of different positions of the continuous casting billet tend to be balanced.

[0032] By adopting the technical solution of the present invention, the segregation phenomenon at the thickness center of the produced continuous casting billet is effectively improved, the quality of the rolled steel plate is significantly improved, and the cracking problem caused by the product due to composition difference is avoided.

[0033] The relevant parameters for the pure iron particle preparation process in the examples are shown in Table 1.

[0034] Table 1. Relevant parameters of the pure iron particle preparation process. The process parameters for converter smelting in the examples and comparative examples are shown in Table 2.

[0035] Table 2 Continuous casting process parameters Table 3 shows the segregation index of the continuously cast billet composition and related parameters of the examples and comparative examples.

[0036] Table 3. Relevant parameters of continuous casting billet and product performance In summary, as can be seen from the table, the segregation index of easily segregated elements is effectively improved when using this method to produce low alloy steel billets, the segregation phenomenon in the thickness direction of the steel plate is significantly reduced, the processing performance of the steel plate is good, and there is no delamination phenomenon due to internal component segregation, which improves the stability of product quality and reduces production costs.

[0037] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving segregation in low-alloy steel billets, characterized in that, The method involves adding material to the surface of the molten steel in the crystallizer during continuous casting: a mixture of pure iron particles and protective slag is added to the central region of the continuous casting billet, wherein the pure iron particles contain nanoparticles, and the nanoparticles are one or both of TiN and TiC; protective slag is added to other regions of the continuous casting billet.

2. The method for improving segregation in low-alloy steel billets according to claim 1, characterized in that, The charging process takes place within 30 minutes before continuous casting begins. The superheat of the molten steel is controlled at 18–22°C, and the billet pulling speed is 0.9–1.1 m / min.

3. The method for improving segregation in low-alloy steel billets according to claim 1, characterized in that, In the mixture of pure iron particles and protective slag, the pure iron particles and protective slag are mixed at a weight percentage of (6-8):

1.

4. The method for improving segregation in low-alloy steel billets according to claim 1, characterized in that, The thickness of the continuously cast billet is 200-300 mm, the width is 1000-2200 mm, and the thickness center area refers to the area 10-20 mm above and below the thickness center line.

5. The method for improving segregation in low-alloy steel billets according to claim 1, characterized in that, The feeding height is 400-600mm from the molten steel surface in the crystallizer; the feeding speed is 0.5-0.7kg / t in the center of the continuous casting billet thickness area and 0.40-0.48kg / t in other areas.

6. The method for improving segregation in low-alloy steel billets according to claim 1, characterized in that, The preparation process of the pure iron particles is as follows: smelting is carried out in a vacuum induction furnace, using industrial pure iron as the raw material, and adding one or both of the following nano-powders: TiN or TiC with a particle size of 0.3-0.6 μm. The amount of nano-powder added accounts for 1 wt% to 2 wt% of the molten steel. Before melting begins, a vacuum operation is performed, followed by electric melting. After melting, the melt temperature is raised to 1636-1736℃, and atomization granulation is performed. The atomization nozzle diameter is 4-6 mm, the blowing gas is nitrogen, the blowing gas temperature is -30-40℃, the blowing gas pressure is 0.2-0.4 MPa, and the steel atomization speed is 6-10 kg / min. Pure iron particles with a particle size of 1-4 mm are then screened out.

7. The method for improving segregation in low-alloy steel billets according to claim 6, characterized in that, The chemical composition of the industrial pure iron, by weight percentage, is as follows: C: 0.005%–0.010%, Si: 0.008%–0.015%, Mn: 0.01%–0.03%, P: 0.002%–0.004%, S: 0.001%–0.003%, with the remainder being Fe and unavoidable impurities.

8. An apparatus used in a method for improving segregation in low-alloy steel billets as described in any one of claims 1-7, characterized in that, It includes a crystallizer, a No. 1 slag feeder, and a No. 2 slag feeder. The No. 1 slag feeder and the No. 2 slag feeder are fixed together side by side by welding the outer shell and are driven by a motor to move horizontally back and forth above the crystallizer. The No. 2 slag feeder feeds material above the center area of ​​the thickness of the continuously cast billet, and the No. 1 slag feeder feeds material to other parts of the liquid surface of the crystallizer.

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