A method for controlling the surface quality of continuous casting slabs

Abstract

The present invention relates to a method for controlling the surface quality of continuous casting billets, and belongs to the field of metallurgical continuous casting technology. In order to solve the problem that crack defects easily appear on the surface of continuous casting billets, the present invention provides a method for controlling the surface quality of continuous casting billets, including the control of continuous casting process parameters, the control of continuous casting tool settings and refined operation management methods, including the control of continuous casting speed, crystallizer water volume, water inlet pressure, inlet and outlet water temperature difference, water inlet temperature, secondary cooling water volume, cooling distribution ratio, first and last stirring parameters; the settings of the crystallizer, full roller cooling zone, secondary cooling zone and straightening machine; the control of crystallizer soft water and water inlet immersion depth. The present invention achieves "zero defects" on the surface of the billet by strictly controlling the continuous casting process parameters, continuous casting tool settings and refined operation management methods, and ultimately achieves the beneficial effects of improving the yield rate, simplifying the cleaning process, reducing costs, improving production efficiency, and reducing the scrap rate.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical continuous casting, and in particular relates to a method for controlling the surface quality of a continuous casting billet. Background Art

[0002] The surface quality of the continuous casting billet determines whether the continuous casting billet needs to be finished before heating. It is an important factor affecting metal yield and cost. It is also the prerequisite for hot delivery and direct rolling of the continuous casting billet. Whether it is hot charging or cold charging and rolling, the surface quality of the continuous casting billet is required to be close to zero defects.

[0003] However, in actual production, due to factors such as instability in the production process, variations in equipment precision, and fluctuations in the continuous casting cooling system, cracks are prone to appearing on the surface of continuous casting billets. According to relevant statistics, 50% of billet defects are surface cracks, which ultimately affect the billet yield and the yield rate of rolled products. Billets with surface cracks require grinding before rolling, which not only causes metal loss but also increases production steps and costs, delaying product delivery, a major problem plaguing various production plants. Summary of the Invention

[0004] In order to solve the problem that crack defects are easily generated on the surface of a continuous casting billet, the present invention provides a method for controlling the surface quality of the continuous casting billet.

[0005] The technical solution of the present invention:

[0006] A method for controlling the surface quality of a continuous casting billet, comprising controlling continuous casting process parameters, controlling the setting of continuous casting tooling, and implementing a refined operation management method;

[0007] The control of the continuous casting process parameters includes controlling the continuous casting speed to be 0.65-0.74 m / min, the water volume of the crystallizer to be 2350 L / min, the water inlet pressure of the crystallizer to be not less than 0.9 MPa, the temperature difference between the inlet and outlet water of the crystallizer to be 6-7.5°C, the water inlet temperature to be 24-28°C, the secondary cooling water volume to be 0.24 L / kg, the secondary cooling to be divided into three zones, and the cooling distribution ratio to be 45 / 35 / 20% in descending order; the first stirring parameter to be 300A / 2Hz, and the final stirring parameter to be 200A / 8Hz;

[0008] The continuous casting adopts a five-machine, five-strand rectangular continuous casting machine equipped with a crystallizer, a foot roller cooling zone, a secondary cooling zone, and a straightening machine. The control of the continuous casting tooling settings includes the settings of the crystallizer, the foot roller cooling zone, the secondary cooling zone, and the straightening machine, including the crystallizer water gap size of 4±0.2mm, 3 rows of cooling rings in the foot roller cooling zone, 17 spray rings in the secondary cooling zone, and 7 continuous straightening and straightening machines per strand;

[0009] The refined operation management method includes controlling the soft water in the crystallizer and the immersion depth of the water inlet.

[0010] Furthermore, the full arc radius of the five-machine five-strand rectangular continuous casting machine is 10.25m, and the production section is 250*280mm.

[0011] Furthermore, the crystallizer liquid level fluctuation is controlled within ±3mm, the water inlet immersion depth is controlled within 110±10mm, and the water flow rate is calculated to be 9.8m according to the crystallizer water gap size. 3 / s, and the soft water inlet pressure of the crystallizer does not exceed 0.95Mpa to prevent eddy currents from forming in the crystallizer and affecting the uniform cooling of the crystallizer. Furthermore, the crystallizer copper tube is fired 0 to 200 times, and no Cr coating above the meniscus falls off. The taper of the copper tube bottom is 1.24% / m.

[0012] Furthermore, the vibration parameters of the crystallizer adopt a non-sinusoidal mode, in which the negative slip time is greater than 24s and the positive slip time is controlled at 15s, and the vibration horizontality and polarization: the horizontal error is ≤5‰ polarization, along the inner and outer arc directions: <0.2mm; horizontally: <0.2mm.

[0013] Furthermore, the foot roller arc accuracy of the foot roller cooling zone is -0.4mm, and two nozzles are set on each surface of the cooling ring. The nozzle opening angle is 80° on the wide side and 70° on the narrow side, which can cover the entire end face of the ingot, and the cooling method is full water cooling.

[0014] Furthermore, the control of the straightening and leveling machines during the production process includes measuring the temperatures of the castings at the 1st, 3rd, 6th and 7th straightening machines to ensure that the straightening temperature is not lower than 980°C.

[0015] Furthermore, the standard requirements for turbid circulating water in the crystallizer soft water are turbidity ≤ 30NTU, calcium hardness ≤ 400mg / L, and the standard requirements for soft circulating water are: pH 7-9, turbidity ≤ 20NTU, calcium hardness ≤ 30mg / L.

[0016] Furthermore, the water inlet temperature of the crystallizer before pouring is not lower than 25°C, and the water inlet temperature during continuous pouring is not lower than 28°C.

[0017] Furthermore, the continuous casting process is carried out under argon protection.

[0018] Beneficial effects of the present invention:

[0019] The method for controlling the surface quality of the continuous casting billet provided by the present invention effectively reduces the probability of crack defects in the continuous casting billet by strictly controlling the continuous casting process parameters, the continuous casting tooling settings and the refined operation management methods, thereby achieving "zero defects" on the surface of the billet, and ultimately achieving the beneficial effects of improving the yield rate, simplifying the cleaning process, reducing costs, improving production efficiency, and reducing the scrap rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a surface photograph of the continuous casting slab obtained in Example 2;

[0021] Figure 2 This is a surface photograph of the continuous casting billet obtained in Example 3. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further described below with reference to the embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be included in the scope of protection of the present invention. The process equipment or devices not specifically noted in the following examples are all conventional equipment or devices in the art. Unless otherwise specified, the raw materials used in the examples of the present invention can be obtained commercially; unless otherwise specified, the technical means used in the examples of the present invention are all conventional means well known to those skilled in the art.

[0023] Example 1

[0024] This embodiment provides a method for controlling the surface quality of a continuous casting billet, including the control of continuous casting process parameters, the control of continuous casting tooling settings, and a refined operation management method.

[0025] This example uses a five-machine, five-strand rectangular continuous casting machine with a cross-section of 250 x 280 mm and a full arc radius of 10.25 m. Regular verification of the large arc is performed for continuous casting, with a standard requirement of -0.5 mm for crack-prone steel grades. This prevents excessive resistance during casting, which could cause cracks in the cast slab. The continuous casting support rollers are super-flattened to ensure smooth rotation and prevent water leaks in the roller cooling system, which could cause casting failures.

[0026] The five-machine, five-strand rectangular continuous casting machine of this embodiment is equipped with a crystallizer, a foot roller cooling zone and a secondary cooling zone. The control of the continuous casting tooling settings includes the settings of the crystallizer, the foot roller cooling zone, the secondary cooling zone and the straightening machine.

[0027] The crystallizer is a continuous casting device that ensures the initial solidification of molten steel and forms a shell of a specified shape. It is the "heart" of the continuous casting machine, and the cooling water in the crystallizer is a key factor in achieving this function. Insufficient cooling water in the crystallizer or excessively high cooling water temperature entering the crystallizer will lead to insufficient cooling capacity, which in turn will cause the shell of the product to be too thin. Under the influence of tensile stress, static pressure of the molten steel, and friction, the weak shell can easily be pulled through, resulting in steel leakage accidents. Conversely, excessive cooling water will lead to excessive cooling, which will cause uneven solidification of the shell and large shell shrinkage, both of which will cause cracks in the cast billet.

[0028] The water gap size of the crystallizer in this embodiment is 4±0.2mm, which ensures uniform water gap and thus ensures uniform cooling of the crystallizer. The water flow rate calculated based on the water gap size of the crystallizer is 9.8m 3 / s, the soft water inlet pressure of the crystallizer shall not exceed 0.95Mpa to prevent eddy currents from forming in the crystallizer and affecting the uniform cooling of the crystallizer.

[0029] Crystallizer water flow rate: controlled at 2350L / min with fluctuation control at the lower limit of -20L / min. The temperature difference of each flow of crystallizer water is controlled at 6-7℃.

[0030] In this embodiment, the vibration parameters of the crystallizer are set in a non-sinusoidal mode, with a negative slip time of >24s and a positive slip time of 15s. The maximum vibration speed of the crystallizer must be greater than the corresponding casting speed to prevent the casting from adhering to the inner wall of the crystallizer during the casting process, making demolding difficult. Vibration horizontality and polarization: Horizontal error ≤ 5‰ polarization, along the inner and outer arc directions: <0.2mm; horizontal direction: <0.2mm. Matching vibration parameters can meet process requirements, the vibration lines are stable, and the casting and crystallizer can be smoothly demolded to prevent adhesion and crack defects.

[0031] The fluctuation of the liquid level in the crystallizer also has a great influence on the quality of the ingot. When the liquid level fluctuates too much, the slag liquid level will fluctuate violently, and even cause slag rolling. The entangled protective slag is generally large in size and has no time to float up, which has a great impact on the internal quality of the ingot. Secondly, large fluctuations in the liquid level will also cause the liquid slag to not flow evenly through the meniscus, resulting in slag-free phenomenon in some parts of the ingot. The heat conduction and lubrication here are not good, and the ingot may even stick to the crystallizer. In addition, if the liquid level fluctuates greatly, it will cause greater erosion of the primary ingot shell, resulting in uneven shell thickness and increased crack sensitivity of the ingot.

[0032] During the pouring process, the cesium source is purged every three casts, and mold level fluctuations are controlled within ±3mm to prevent slag roll-up and cracks. Before startup, the cesium source is cleaned of accumulated slag, and the liquid level is calibrated to "empty." The height of the new ladle nozzle is also measured. Consistent cesium source feedback is ensured for each flow, ensuring consistent mold nozzle immersion depth. Controlling the nozzle immersion depth within 110±10mm ensures good mold protection slag slagging, significantly preventing cracks in the cast billet during the lubrication process.

[0033] Before the mold is put into production, the inner wall of the copper tube must be inspected. Any signs of chromium plating shedding or severe wear are prohibited. The mold copper tube must be used for 0 to 200 burns, with no chromium plating shedding above the meniscus and a taper of 1.24% / m at the bottom end. If the mold exceeds 200 burns, it must be taken offline for an overhaul. The inner and outer walls of the copper tube and the foot roller section must be inspected, and any defects that do not meet process requirements must be adjusted.

[0034] In this embodiment, the foot roller cooling zone is provided with three rows of cooling rings, and two nozzles are set on each surface of the cooling ring. The nozzle opening angle is 80° on the wide side and 70° on the narrow side, which can cover the entire end face of the ingot, and the cooling method is full water cooling.

[0035] This embodiment utilizes a new arc alignment method in the foot roll cooling zone, ensuring an arc accuracy of 0.4 mm. Arc alignment is performed at three points across the cross section to prevent foot roll deflection. The spray ring in the foot roll cooling zone is aligned, the nozzles are in good condition, and the nozzle angle is not blocked by the foot roll. Accuracy is guaranteed. Foot roll nozzles are regularly replaced, and the spray status is checked during production to ensure alignment meets process requirements during casting and uniform cooling in the foot roll cooling zone.

[0036] The secondary cooling system significantly impacts the quality of the ingot. Excessive secondary cooling intensity can lead to a sudden drop in the ingot's surface temperature. This significant volumetric shrinkage can lead to significant thermal stress, which can concentrate stress in weak areas of the shell and cause cracks. If microcracks exist before the secondary cooling process, they can propagate in areas with greater secondary cooling intensity. Uneven secondary cooling intensity can generate greater thermal stress in areas with faster and slower cooling, further increasing the likelihood of cracks. Weak secondary cooling intensity can lead to higher surface temperatures and a softer shell, exacerbating the tendency for bulging. One of the most common crack-prone conditions during the secondary cooling stage is straightening in the third brittle zone of a steel grade, where the steel's inherent resistance is poor. Applying additional straightening stress can easily reach the ultimate stress, leading to ingot cracking.

[0037] In the present embodiment, 17 spray rings are provided in the secondary cooling zone, and the spacing between the spray rings increases successively, which effectively controls the occurrence of cracks caused by repeated temperature regeneration during the cooling process of the casting.

[0038] This embodiment of the continuous straightening and straightening machine consists of seven units per stream. Control of the straightening and straightening machines during production involves measuring the temperatures of the castings at the first, third, sixth, and seventh straightening machines to ensure the straightening temperature is no less than 980°C, avoiding the third brittle zone during cooling, which can cause transverse cracks. The straightening machines, especially the straightening units, are prevented from deviating during operation. The arch-type multi-point straightening machine uses multiple points to prevent cracks caused by excessive stress in the castings.

[0039] The control method for the continuous casting process parameters in this embodiment is as follows:

[0040] According to the actual production rhythm on site, 100t of molten steel is applicable. The continuous casting process is carried out under argon protection, which effectively controls the secondary oxidation of the molten steel caused by the absorption of oxygen in the air during the pouring process.

[0041] For the control of continuous casting process parameters in this embodiment, the continuous casting speed is controlled to be 0.65~0.74m / min, the water volume of the crystallizer is controlled to be 2350L / min, the water inlet pressure of the crystallizer is not less than 0.9MPa, the temperature difference between the inlet and outlet water of the crystallizer is 6~7.5℃, the inlet water temperature is 24~28℃, the secondary cooling water volume is 0.24L / kg, the secondary cooling is divided into 3 areas, and the cooling distribution ratio is 45 / 35 / 20% in descending order; the first stirring parameter is 300 / 2Hz, and the final stirring parameter is 200A / 8Hz.

[0042] The overall parameters of this embodiment are well matched, and the pouring time can be effectively controlled. While ensuring the production rhythm, it can also ensure that the surface crack defects caused by repeated temperature recovery of the ingot are prevented.

[0043] Drawing speed and temperature are important factors in controlling the quality of the billet, which can ensure stability during the drawing process and stabilize the billet quality without causing large quality fluctuations;

[0044] The pulling speed of the billet determines the production efficiency of the continuous casting machine, but high pulling speed and high billet quality are often contradictory. Increasing the pulling speed will bring a series of effects, such as causing flow field disorder in the crystallizer, large fluctuations in the liquid level in the crystallizer, and a series of other problems. The most serious is that increasing the pulling speed will cause the billet shell out of the crystallizer to be too thin, the billet shell temperature is high, making the billet shell relatively weak, prone to cracks and even steel leakage accidents.

[0045] In this embodiment, based on a casting speed of 0.65-0.74 m / min, the mold cooling water can fully meet the formation of the primary shell, while also ensuring uniform cooling of the mold. The difference between the actual casting speed and the set casting speed is regularly tested, and the standard requirement is ±0.01 m / min.

[0046] The secondary cooling water volume in this embodiment is 0.24 L / kg. The lower water volume keeps the billet in a high temperature section to control the billet temperature recovery and cause thermal stress to generate internal cracks.

[0047] In this embodiment, the three-stage cooling distribution ratio of the secondary cooling zone is 45 / 35 / 20% in descending order, which can uniformly cool the ingot and prevent crack defects caused by repeated reheating of the ingot.

[0048] The superheat of molten steel is defined as the difference between the casting temperature and the temperature of the solidus line of the steel grade. For a given steel grade, the solidus line temperature of the molten steel hardly changes, so the casting temperature is the main factor affecting the superheat. If the superheat of the molten steel is too high, the thickness of the billet shell after leaving the crystallizer will be too thin. In addition, the movement of the molten steel caused by the high temperature intensifies the scouring of the dendrites and the billet shell. In addition, the heating effect of the molten steel on the billet shell will further thin the billet shell, which seriously increases the possibility of cracking of the billet shell (mainly longitudinal cracking). In the continuous casting process of this embodiment, the superheat of the first furnace is ≤40°C, and the continuous casting furnace is ≤35°C, to ensure constant temperature and speed during the casting process.

[0049] During continuous casting, the protective pouring and the connection of the ladle drainage sand should be done well. The drainage sand connection operation should be carried out before the ladle pouring to effectively control the drainage sand from entering the tundish and prevent the drainage sand from entering the tundish and causing secondary contamination of the molten steel.

[0050] Head-end electromagnetic stirring stirs the molten steel entering the mold. Its function is to even out the temperature of the molten steel, promote the growth of equiaxed grains, and complete the uniform formation of the primary shell. This embodiment uses head-end electromagnetic stirring. Based on the equipment's function, the head-end electromagnetic stirring is set to a maximum stirring intensity of >300 Gs. This effectively stirs the molten steel to even out the temperature and promote the formation of a chill layer in the cast strand.

[0051] The refined operation management method of this embodiment is also reflected in the inspection of the continuous casting water quality. The standard requirements for turbid ring water in the soft water of the crystallizer are turbidity ≤30NTU, calcium hardness ≤400mg / L, and the standard requirements for soft ring water are: PH7~9, turbidity ≤20NTU, calcium hardness ≤30mg / L. Controlling the continuous casting water quality on this basis ensures that the activity of cooling water can meet the process requirements, effectively control the uniformity of the cooling process of the ingot, and prevent the occurrence of stress cracks. The water inlet temperature of the crystallizer before pouring is not lower than 25°C, and the water inlet temperature during the continuous pouring process is not lower than 28°C, which can ensure the uniform cooling of the crystallizer once and the cooling of the full roller section. Effectively control the probability of cracks.

[0052] Example 2

[0053] This embodiment provides a method for controlling the surface quality of a continuous casting billet, including the control of continuous casting process parameters, the control of continuous casting tooling settings, and a refined operation management method.

[0054] The chemical composition of the continuous casting billet in this embodiment includes, by weight percentage, C: 0.21%, Si: 0.23%, Mn: 0.40%, P≤0.013%, S≤0.005%, Al: 0.012%, and the rest is Fe and unavoidable impurities.

[0055] The only difference between this embodiment and Example 1 is that the continuous casting process parameters in this embodiment are controlled as follows: casting speed 0.74m / min, crystallizer water volume 2350L / min, fluctuation -20L / min, specific water volume 0.24L / kg, distribution ratio 45 / 35 / 20%, and first stirring parameter 300A / 2Hz.

[0056] The surface of the continuous casting billet obtained in this embodiment is as follows Figure 1 As shown, the first pass rate of the billet surface is 100%.

[0057] Example 3

[0058] This embodiment provides a method for controlling the surface quality of a continuous casting billet, including the control of continuous casting process parameters, the control of continuous casting tooling settings, and a refined operation management method.

[0059] The chemical composition of the continuous casting billet in this embodiment includes, by weight percentage, C: 0.40%, Si: 0.23%, Mn: 0.40%, P≤0.013%, S: 0.55%, Al: 0.012%, and the rest is Fe and unavoidable impurities.

[0060] The only difference between this embodiment and embodiment 1 is that the continuous casting process parameters are controlled as follows: casting speed 0.65m / min, crystallizer water volume 2350L / min, fluctuation -20L / min, specific water volume 0.20L / kg, distribution ratio 45 / 35 / 20%, and first stirring parameter 300A / 2Hz.

[0061] The surface of the continuous casting billet obtained in this embodiment is as follows Figure 2 As shown, the first pass rate of the billet surface is 100%.

Claims

1. A method for controlling the surface quality of a continuous casting billet, characterized in that: Including the control of continuous casting process parameters, the control of continuous casting tooling settings and refined operation management methods; The control of the continuous casting process parameters includes controlling the continuous casting speed to 0.65-0.74 m / min, controlling the water volume of the crystallizer to 2350 L / min, the water inlet temperature of the crystallizer before pouring is not less than 25°C, the water inlet temperature during the continuous casting process is not less than 28°C, and the temperature difference between the inlet and outlet water of the crystallizer is 6-7.5°C. The vibration parameters of the crystallizer adopt a non-sinusoidal mode, in which the negative slip time is greater than 24s and the positive slip time is controlled within 15s. The vibration horizontality and polarization: the horizontal error is ≤5‰ polarization, along the inner and outer arc directions: <0.2mm; Horizontal: <0.2mm; The secondary cooling water volume is 0.24L / kg, and the secondary cooling is divided into three zones with a cooling distribution ratio of 45 / 35 / 20% in descending order. The first stirring parameter is 300 / 2Hz, and the final stirring parameter is 200A / 8Hz. The continuous casting adopts a five-machine, five-strand rectangular continuous casting machine equipped with a crystallizer, a foot roller cooling zone, a secondary cooling zone, and a straightening machine. The control of the continuous casting tooling settings includes the settings of the crystallizer, the foot roller cooling zone, the secondary cooling zone, and the straightening machine, including the crystallizer water gap size of 4±0.2mm, 3 rows of cooling rings in the foot roller cooling zone, 17 spray rings in the secondary cooling zone, and 7 continuous straightening and straightening machines per strand; The foot roller arc accuracy of the foot roller cooling zone is -0.4mm. Two nozzles are set on each surface of the cooling ring. The nozzle opening angle is 80° on the wide side and 70° on the narrow side, which can cover the entire end face of the ingot. The cooling method is full water cooling. The control of the straightening and leveling machines during production includes measuring the temperature of the billets at the front, front, rear and rear straightening machines at 1st, 3rd, 6th and 7th stands to ensure that the straightening temperature is not lower than 980°C. The refined operation management method includes the control of the crystallizer soft water and the immersion depth of the nozzle. The crystallizer liquid level fluctuation is controlled within ±3mm, the nozzle immersion depth is controlled within 110±10mm, and the water flow rate is calculated based on the crystallizer water gap size to be 9.8m 3 / s, the soft water inlet pressure of the crystallizer shall not exceed 0.95Mpa and shall not be less than 0.9MPa; The standard requirements for turbid circulating water in the crystallizer soft water are turbidity ≤30NTU, calcium hardness ≤400mg / L, and the standard requirements for soft circulating water are: pH 7~9, turbidity ≤20NTU, calcium hardness ≤30mg / L.

2. A method for controlling the surface quality of a continuous casting billet according to claim 1, characterized in that: The full arc radius of the five-machine, five-strand rectangular continuous casting machine is 10.25m, and the production section is 250*280mm.

3. The method for controlling the surface quality of a continuous casting billet according to claim 2, characterized in that: The crystallizer copper tube was fired at 0 to 200 times, and there was no Cr coating falling off above the meniscus. The taper of the bottom of the copper tube was 1.24% / m.

4. A method for controlling the surface quality of a continuous casting billet according to claim 3, characterized in that: The continuous casting process is carried out under argon protection.

Images