Method for controlling surface quality of bridge cable steel continuous cast billet

Abstract

The present invention relates to a method for controlling the surface quality of continuous casting billets of bridge cable steel, comprising: controlling the composition of mold powder, wherein the composition of the mold powder, in terms of mass percentage, comprises: 28~30% of SiO₂, 21~28% of CaO, 0.7~0.9% of MgO, ≤3.0% of Al₂O₃, 1~1.5% of B₂O₃, 8~10% of Na₂O, 6~8% of F, 15~18% of TC, 0.5~1% of Li₂O, ≤0.5% of moisture, with the balance being unavoidable impurities; fine management of copper tubes, wherein the copper tubes do not meet the scrap judgment standard, when the steel passing amount of the copper tube is ≤4000t, the basicity R of the mold powder is controlled to be 0.7≤R≤0.8, and 17%<TC≤18%; when 4000t < the steel passing amount of the copper tube ≤8000t, the basicity R of the mold powder is controlled to be 0.8<R≤0.9, and 16%<TC≤17%; when 8000t < the steel passing amount of the copper tube ≤12000t, the basicity R of the mold powder is controlled to be 0.9<R≤1.0, and 15%≤TC≤16%; controlling the heat flux density of the crystallizer, controlling the superheat degree of molten steel and the water amount of the crystallizer to stabilize the heat flux density, and controlling the range of heat flux density ΔΦ≤0.03MW / m 2 . The present invention effectively solves the problem of poor surface quality of continuous casting billets caused by insufficient heat transfer control and lubrication capacity in the later service period of copper tubes.

Description

Technical Field

[0001] This invention relates to a method for controlling the surface quality of continuously cast steel billets for bridge cables, belonging to the technical field of continuous casting. Background Technology

[0002] Bridge cables are the core load-bearing components of long-span bridges such as cable-stayed bridges and suspension bridges. With the increasing demand for longer spans and heavier loads in bridges, bridge cable wires are also developing towards larger specifications and higher strength. To improve the strength of bridge cable wires, the content of elements such as C, Si, Mn, and V in the initial casting billets required for their production is constantly increasing, further complicating the control of the surface quality of the continuously cast billets.

[0003] The patent application published as CN112322865A addresses the surface quality problem of cast billets through shot blasting and surface grinding, effectively improving the yield of rolled steel. However, this is a reactive measure and does not fundamentally solve the surface quality problem of cast billets. Furthermore, the yield of continuous casting is low, the production process is complex, and production efficiency is greatly reduced.

[0004] The patent application published as CN118875234A controls the crystallization rate of the protective slag by optimizing the composition and production process of the protective slag, increases the thermal resistance of the slag film, achieves the effect of slow cooling of the billet, and reduces the generation of cracks. However, it ignores the influence of deformation and wear of the copper tube of the crystallizer on heat transfer.

[0005] Existing technologies improve the surface quality of cast billets through surface grinding, but this significantly reduces the continuous casting yield and overall production efficiency. Improper grinding may expose internal quality problems, failing to fundamentally solve the surface quality issues of bridge cable steel cast billets. To address these problems, this invention provides a method for controlling the surface quality of continuously cast bridge cable steel billets, effectively improving the surface quality of the cast billets. Summary of the Invention

[0006] To address the aforementioned problems, this invention discloses a method for controlling the surface quality of continuously cast steel billets for bridge cables, resolving issues such as irregular vibration marks, bubbles, and cracks on the surface of these billets. The specific technical solution is as follows: A method for controlling the surface quality of continuously cast steel billets for bridge cables includes control of the composition of the protective slag, refined management of the copper tube, and control of the heat flux density of the crystallizer. The protective slag covers the meniscus of the molten steel inside the copper tube. The molten steel enters the copper tube through a submerged entry nozzle, the outlet of which is located 90-110 mm below the protective slag. The copper tube penetrates the crystallizer, forming a casting channel. A water cooling system is installed around the outside of the copper tube. The steel billet exiting the copper tube enters the continuous casting machine, and the continuous casting machine yields the continuously cast steel billet for bridge cables. One casting cycle in the crystallizer consists of one or more continuous castings of molten steel from a ladle. Before casting begins, a protective slag with the corresponding composition is selected for protective casting. Control of mold powder composition: the mold powder comprises the following components in mass percentage: 28~30% of SiO2, 21~28% of CaO, 0.7~0.9% of MgO, ≤3.0% of Al2O3, 1~1.5% of B2O3, 8~10% of Na2O, 6~8% of F, 15~18% of TC, 0.5~1% of Li2O, ≤0.5% of moisture, with the balance being unavoidable impurities; Refined management of copper tube: first determine whether the copper tube meets the scrapping standard, replace with a new copper tube if it meets the scrapping standard; if the copper tube does not meet the scrapping standard, select the basicity and carbon content of the mold powder according to the steel passing amount of the copper tube. When the steel passing amount of the copper tube is ≤4000t, the basicity R of the mold powder is controlled at 0.7≤R≤0.8, and 17%<TC≤18%; when 4000t < the steel passing amount of the copper tube ≤8000t, the basicity R of the mold powder is controlled at 0.8<R≤0.9, and 16%<TC≤17%; when 8000t < the steel passing amount of the copper tube ≤12000t, the basicity R of the mold powder is controlled at 0.9<R≤1.0, and 15%≤TC≤16%; Control of mold heat flux density: the formula for mold heat flux density Φ is Φ=Q×C×ΔT / S, in the formula: Φ is the mold heat flux density, W / m 2 ; Q is the water flow rate of the mold, L / s; C is the specific heat capacity of water, which is 4200J / (kg·℃); ΔT is the temperature difference between inlet and outlet water of the mold, ℃; S is the effective area of the mold, m 2 ; according to the formula, the mold heat flux density Φ can be controlled by controlling the temperature difference ΔT between inlet and outlet water of the mold and the water flow rate Q of the mold; controlling the range difference ΔΦ of heat flux density of the whole casting sequence in the mold to ΔΦ≤0.03MW / m 2 , wherein ΔΦ is the range difference of heat flux density of the whole casting sequence in the mold, that is ΔΦ=Φmax-Φmin, MW / min.

[0007] Further, the scrapping standards for the copper tube are: ① the steel passing amount of the copper tube > 12000t; ② the wear amount of the lower opening of the copper tube > 0.8mm; ③ the coating peeling at the lower opening of the copper tube > 250mm in length or > 0.8mm in width or the scratch depth > 1mm; ④ there are scratches, coating peeling and plastic deformation within 150mm below the meniscus of the copper tube; the copper tube is scrapped if it meets any one of the scrapping standards.

[0008] Further, the melting point of the mold powder is 1063-1083℃, the viscosity at 1300℃ is 0.3~0.4Pa·s, the crystallinity is 45~65%, the proportion of cuspidine is ≥15%, and the thickness of the mold powder layer is 100~120mm.

[0009] Further, the mold heat flux density Φ is 1.497~1.527MW / m 2The molten steel is heated by induction heating in an tundish to achieve a superheat of 20-23°C. The temperature difference ΔT between the inlet and outlet water of the crystallizer is controlled by adjusting the superheat and the water flow rate. The water flow rate is 2990-3010 L / min, the temperature difference is 7.9-8.0°C, and the effective area of ​​the copper tubes in the crystallizer is 1.103-1.104 m². 2 .

[0010] Furthermore, the continuous casting machine has a cross-sectional dimension of 300mm×390mm, a casting speed of 0.65~0.67m / min, an electromagnetic stirring current of 749~751A in the crystallizer, and a frequency of 1.4~1.6Hz.

[0011] The continuous casting billet for bridge cable steel is prepared by the above-mentioned method for surface quality control of continuous casting billet for bridge cable steel.

[0012] Furthermore, the chemical composition of the bridge cable steel continuous casting billet, by mass percentage, includes: C 0.85~0.96%, Si 0.85~1.30%, Mn 0.71~0.86%, P ≤0.012%, S ≤0.005%, O ≤0.003%, N ≤0.006%, Cr 0.28~0.37%, Ti ≤0.03%, V 0.02~0.08%, Al ≤0.03%, with the balance being Fe and unavoidable impurities.

[0013] Traditional processes select protective slags that match the characteristics of the steel grade for lubrication and heat transfer control. However, they neglect the impact of changes in the taper of the copper tube and plating wear after long-term use on lubrication, heat transfer control, and the inflow of protective slag. Based on the influence of steel throughput on the taper of the copper tube, plating wear, and the inflow of protective slag, protective slag addition schemes have been developed for different service periods of the copper tube. For example, when the steel throughput is ≤4000t, the copper tube taper is large, the billet pulling resistance is high, and the protective slag inflow channel is small. It is necessary to reduce the basicity, increase the carbon content, improve the lubrication performance of the protective slag, and control the inflow of protective slag. When the steel throughput is >8000t, the copper tube taper is small, wear is severe, the billet pulling resistance is low, and heat transfer control is poor. It is necessary to increase the basicity, reduce the carbon content, improve the heat transfer control capacity of the protective slag, and increase the inflow of protective slag.

[0014] This invention establishes a link between the performance of the protective slag and the usage of the copper tubes in the crystallizer. Based on the wear condition of the copper tubes in the crystallizer, appropriate protective slag is added in stages. By optimizing the basicity and carbon content of the protective slag, the heat transfer and lubrication control performance of the protective slag is improved, making up for the insufficient heat transfer and lubrication control capabilities of the copper tubes in the later stages of their service life, and effectively improving the surface quality of the cast billet.

[0015] This invention employs tundish induction heating technology, effectively reducing the superheat of molten steel and controlling the superheat fluctuation throughout the entire casting cycle to within 3°C (the entire casting cycle includes multiple ladles of steel, and the superheat of the molten steel in all ladles is controlled to within 3°C during the multiple ladles of steel. The superheat of each ladle is controlled by induction heating of the molten steel in the tundish). Simultaneously, it strengthens the control of the crystallizer water flow, reduces the fluctuation of the crystallizer heat flux density, improves the cooling uniformity of the cast billet, and effectively improves the surface quality of the cast billet.

[0016] (1) Refined management of copper tubes: the copper tubes of the crystallizer are adapted to protective slags with different properties under different working conditions, which effectively solves the problem of poor heat transfer caused by copper tube wear and deformation.

[0017] (2) Optimize the performance of protective slag. Replace part of Na2O with Li2O. On the one hand, reduce the impact of increased alkalinity on melting point. On the other hand, increase the proportion of gun crystals and enhance the heat transfer control capability of protective slag, which is conducive to improving the heat transfer uniformity of copper tubes in the later stage of service.

[0018] (3) Heat flux density control: Induction heating technology in the tundish is adopted to accurately control the superheat of the molten steel in the tundish, and the water flow control in the crystallizer is strengthened to ensure that the heat flux density difference of the crystallizer throughout the casting is ≤0.03MW / m 2 This effectively improves the uniformity of heat transfer in copper tubes. Detailed Implementation

[0019] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0020] Example 1

[0021] The steel grade is SRWRS87MnSi, with a C content of 0.85%. The furnace capacity of the converter and LF refining furnace is 135t, and the cross-sectional dimensions of the 5-strand continuous casting machine are 300mm×390mm.

[0022] (1) The pulling speed is controlled at 0.65m / min, the electromagnetic stirring current of the crystallizer is 749A and the frequency is 1.4Hz.

[0023] (2) The heat flux density of the crystallizer is 1.497 MW / m 2 The tundish superheat is 20℃, the crystallizer water temperature difference is 7.9℃, the crystallizer water flow rate is 2990L / min, and the effective area of ​​the copper tubes in the crystallizer is 1.104m². 2 .

[0024] (3) The composition of the protective slag by mass percentage includes: SiO2 30%, CaO 21%, MgO 0.7%, Al2O3 3.0%, B2O3 1.0%, Na2O 10%, F 6%, TC 18.0%, Li2O 0.5%, moisture 0.5%, and the balance being unavoidable impurities.

[0025] (4) The copper tube of the crystallizer is worn at the bottom with a wear of 0.4 mm, the coating peels off at the bottom with a length of 100 mm and a width of 0.3 mm, there are no scratches on the surface, the steel throughput is 4000 t, the basicity of the protective slag is 0.7, and the C content of the protective slag is 18%.

[0026] (5) The melting point of the protective slag is 1083℃, the viscosity at 1300℃ is 0.4Pa·S, the crystallization rate is 45%, the proportion of gun crystal is 15%, and the thickness of the protective slag is 100mm.

[0027] (6) The composition of the cable steel by mass percentage includes: C 0.85%, Si 0.85%, Mn 0.71%, P 0.012%, S 0.005%, O 0.003%, N 0.006%, Ti 0.03%, Cr 0.28%, V 0.08%, Al 0.03%, with the balance being Fe and unavoidable impurities.

[0028] Example 2

[0029] The steel grade is SRWRS92Si, the C content is 0.92%, the furnace capacity of the converter and LF refining furnace is 135t, and the cross-sectional dimensions of the 5-strand continuous casting machine are 300mm×390mm.

[0030] (1) Control the pulling speed to 0.66m / min, and the current of the electromagnetic stirring of the crystallizer to 750A and the frequency to 1.5Hz.

[0031] (2) The heat flux density of the crystallizer is 1.502 MW / m 2 The tundish superheat is 22℃, the crystallizer water temperature difference is 7.9℃, the crystallizer water flow rate is 2995L / min, and the effective area of ​​the crystallizer copper tubes is 1.103m². 2 .

[0032] (3) The composition of the protective slag by mass percentage includes: 29% SiO2, 25% CaO, 0.8% MgO, 2.5% Al2O3, 1.0% B2O3, 9% Na2O, 7% F, 16.5% TC, 0.8% Li2O, 0.4% moisture, and the balance being unavoidable impurities.

[0033] (4) The copper tube of the crystallizer is worn at the bottom with a wear of 0.6 mm, the coating peels off at the bottom with a length of 180 mm and a width of 0.5 mm, there are no scratches on the surface, the steel throughput is 8000 t, the basicity of the protective slag is 0.86, and the C content of the protective slag is 16.5%.

[0034] (5) The melting point of the protective slag is 1073℃, the viscosity at 1300℃ is 0.35Pa·S, the crystallization rate is 50%, the proportion of gun crystal is 20%, and the thickness of the protective slag is 110mm.

[0035] (6) The composition of the cable steel by mass percentage includes: C 0.92%, Si 1.12%, Mn 0.78%, P 0.010%, S 0.004%, O 0.0025%, N 0.0055%, Ti 0.02%, Cr 0.33%, V 0.04%, Al 0.02%, with the balance being Fe and unavoidable impurities.

[0036] Example 3

[0037] The steel grade is SRWRS96Si, with a C content of 0.96%. The furnace capacity of the converter and LF refining furnace is 135t, and the cross-sectional dimensions of the 5-strand continuous casting machine are 300mm×390mm.

[0038] (1) The pulling speed is controlled at 0.67 m / min, the electromagnetic stirring current of the crystallizer is 751 A and the frequency is 1.6 Hz.

[0039] (2) The heat flux density of the crystallizer is 1.527 MW / m 2 The tundish superheat is 23℃, the crystallizer water temperature difference is 8.0℃, the crystallizer water flow rate is 3010L / min, and the effective area of ​​the crystallizer copper tubes is 1.103m². 2 .

[0040] (3) The composition of the protective slag by mass percentage includes: 28% SiO2, 28% CaO, 0.9% MgO, 2.0% Al2O3, 1.5% B2O3, 8% Na2O, 8% F, 15.0% TC, 1% Li2O, 0.3% moisture, and the balance being unavoidable impurities.

[0041] (4) The lower end of the copper tube of the crystallizer is worn by 0.8 mm, the length of the coating peeled off at the lower end is 250 mm and the width is 0.8 mm, there are no scratches on the surface, the steel throughput is 12000 t, the basicity of the protective slag is 1.0, and the C content of the protective slag is 15%.

[0042] (5) The melting point of the protective slag is 1063℃, the viscosity at 1300℃ is 0.3Pa·S, the crystallization rate is 60%, the proportion of gun crystal is 30%, and the thickness of the protective slag is 120mm.

[0043] (6) The composition of the cable steel by mass percentage includes: C 0.96%, Si 1.3%, Mn 0.86%, P 0.012%, S 0.005%, O 0.003%, N 0.006%, Ti 0.03%, Cr 0.37%, V 0.02%, Al 0.03%, with the balance being Fe and unavoidable impurities.

[0044] In Examples 1-3, the surface vibration marks of the large square billet cable steel produced under each working condition were regular, with no bubbles or cracks, effectively improving the surface quality of the large square billet cable steel.

[0045] Comparative Example 1 The method of this comparative example is basically the same as that of Example 1, except that the basicity of the protective slag is 1.0.

[0046] Comparative Example 2 This comparative example is basically the same as the method in Example 2, except that the heat flux density of the crystallizer is 1.543 MW / m³. 2 .

[0047] Comparative Example 3 The method of this comparative example is basically the same as that of Example 3, except that the content of protective slag C is 18%.

[0048] In Comparative Examples 1-3, the surface of the large square billet cable steel produced under each working condition had surface defects such as cracks or bubbles.

[0049] Performance testing The experiment was conducted according to conventional large-scale production. The surface of high-carbon steel small square billets was hot-pickled. The surface defects of the billets under each test scheme were statistically analyzed. The test results of Examples 1-3 and Comparative Examples 1-3 were compared. The test results are shown in Table 1 below.

[0050] Table 1 Performance Test Results

[0051] In summary, (1) the present invention establishes a connection between the performance of the protective slag and the use of the copper tube in the crystallizer, effectively solving the problem of insufficient heat transfer and lubrication control in the later stage of copper tube service, which caused the surface quality of the billet.

[0052] (2) The present invention adopts tundish induction heating technology, which reduces the uneven heat transfer caused by the fluctuation of molten steel superheat, and strengthens the control of crystallizer water volume and temperature difference, effectively controlling the heat flux density of crystallizer.

[0053] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0054] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for controlling the surface quality of continuously cast steel billets for bridge cables, characterized in that, It comprises mold powder composition control, refined copper tube management and mold heat flux density control, wherein the mold powder covers the liquid steel meniscus in the copper tube, molten steel enters the copper tube through an immersion nozzle, and the outlet of the immersion nozzle is located 90-110mm below the mold powder; the copper tube runs through the crystallizer to form a pouring flow channel; a water cooling system is arranged around the outside of the copper tube, and the steel billet coming out of the copper tube enters a continuous casting machine, so that a bridge cable steel continuous casting billet is obtained via the continuous casting machine; One casting sequence of the crystallizer comprises continuous pouring of molten steel from more than one ladle, and a mold powder with corresponding composition is selected for protective pouring before casting starting; Mold powder composition control: in terms of mass percentage, the components of the mold powder comprise: 28~30% of SiO2, 21~28% of CaO, 0.7~0.9% of MgO, Al2O3 ≤3.0%, 1~1.5% of B2O3, 8~10% of Na2O, 6~8% of F, 15~18% of TC, 0.5~1% of Li2O, moisture ≤0.5%, with the balance being unavoidable impurities; Refined copper tube management: first, judging whether the copper tube meets the scrap standard, replacing with a new copper tube if the scrap standard is met; if the copper tube does not meet the scrap standard, the alkalinity and carbon content of the mold powder are selected according to the steel throughput of the copper tube; when the steel throughput of the copper tube is ≤4000t, the alkalinity R of the mold powder is controlled to be 0.7≤R≤0.8, and 17%<TC≤18%; when 4000t < the steel throughput of the copper tube ≤8000t, the alkalinity R of the mold powder is controlled to be 0.8<R≤0.9, and 16%<TC≤17%; when 8000t < the steel throughput of the copper tube ≤12000t, the alkalinity R of the mold powder is controlled to be 0.9<R≤1.0, and 15%≤TC≤16%; Crystallizer heat flux density control: the formula of the crystallizer heat flux density Φ is Φ = Q × C × ΔT / S, wherein: Φ is the crystallizer heat flux density, W / m 2 ; Q is the crystallizer water quantity, L / s; C is the specific heat capacity of water, 4200 J / (kg·℃); ΔT is the crystallizer water temperature difference, ℃; S is the effective area of the crystallizer, m 2 ; according to the formula, the crystallizer heat flux density Φ is controlled by controlling the crystallizer water temperature difference ΔT and the crystallizer water quantity Q; The whole heat flux density difference of the crystallizer is controlled to be less than or equal to 0.03 MW / m 2 Wherein, ΔΦ is the heat flux density difference of the whole casting of the crystallizer, namely ΔΦ=Φmax-Φmin, MW / min.

2. The method for surface quality control of continuous casting billets for bridge cable steel according to claim 1, characterized in that, The copper tube scrap standard is as follows: ① the steel throughput of the copper tube > 12000t; ② the wear amount of the lower opening of the copper tube > 0.8mm; ③ the plating peeling of the lower opening of the copper tube is > 250mm in length or > 0.8mm in width or > 1mm in scratching depth; ④ there are scratches, plating peeling and plastic deformation within 150mm downward from the copper tube meniscus; the copper tube is scrapped if any one of the scrap standards is met.

3. The method for surface quality control of bridge cable steel continuous casting billets according to claim 1, characterized in that, The melting point of the mold powder is 1063-1083℃, the viscosity at 1300℃ is 0.3~0.4Pa·s, the crystallization rate is 45~65%, the proportion of cuspidine is ≥15%, and the thickness of the mold powder layer is 100~120mm.

4. The method for surface quality control of bridge cable steel continuous casting billets according to claim 1, characterized in that, The crystallizer heat flux Φ is 1.497-1.527 MW / m 2 By controlling the superheat of the molten steel and the water quantity of the crystallizer, the temperature difference ΔT of the water in and out of the crystallizer is controlled, the molten steel is inductively heated in the tundish, the superheat of the molten steel is 20-23 ℃, the water quantity of the crystallizer is 2990-3010 L / min, the temperature difference ΔT of the water in and out of the crystallizer is 7.9-8.0 ℃, and the effective area of the copper pipe of the crystallizer is 1.103-1.104 m 2 .

5. The method for surface quality control of bridge cable steel continuous casting billets according to claim 1, characterized in that, The section size of the continuous casting machine is 300mm×390mm, the casting speed is 0.65~0.67m / min, the electromagnetic stirring current of the crystallizer is 749~751A, and the frequency is 1.4~1.6Hz.

6. A bridge cable steel continuous casting billet prepared by the surface quality control method for bridge cable steel continuous casting billet according to any one of claims 1-5.

7. The continuous casting billet for bridge cable steel according to claim 6, characterized in that, The chemical composition of the continuous casting billet for bridge cable steel, by mass percentage, includes: C 0.85~0.96%, Si 0.85~1.30%, Mn 0.71~0.86%, P ≤0.012%, S≤0.005%, O ≤0.003%, N ≤0.006%, Cr 0.28~0.37%, Ti ≤0.03%, V 0.02~0.08%, Al ≤0.03%, with the balance being Fe and unavoidable impurities.

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