Method for overcoming black stripe defect at edge part of stainless steel cold-rolled sheet

Abstract

The invention belongs to the field of steel production, the black stripe defect is a typical quality defect appearing after a ferritic stainless steel cold-rolled plate or a ferritic stainless steel hot-rolled plate with Cr less than 14% is subjected to acid pickling, and the defect comes from the surface of a continuous casting billet and has a direct corresponding relation with continuous casting primary cooling strength and casting powder performance. The invention discloses a method for solving the black stripe defect of the edge of a stainless steel cold-rolled sheet. The black stripe defect of a low-chromium ferrite stainless steel plate is thoroughly solved by newly designing a continuous casting cooling process and casting powder parameters.

Description

Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to a method for solving the black stripe defect at the edge of a stainless steel cold-rolled plate. Background Art

[0002] After the cold-rolled or hot-rolled stainless steel plate is pickled, there are continuous or intermittent black streaks within 200mm from the edge of the steel plate. This defect generally occurs on ferritic stainless steel with a Cr content of less than 14.00%, and it is difficult to eliminate it through cold rolling and pickling, which has a great impact on the appearance quality of the product. After analysis, the C content at the defect is higher than that at the normal matrix. This is mainly because the C in the continuous casting protection slag penetrates into the continuous casting shell and cannot be removed during the steel rolling and pickling process. It remains on the surface of the steel plate and forms a significant color difference with other parts, and appears black under natural light.

[0003] The usual solution is to reduce the C content of the mold slag as much as possible to reduce the carburization of the mold slag into the shell of the ingot, but this will lead to the problem of too fast melting of the mold slag and the thick mold slag, which will cause the ingot slag to roll or the slag strip in the crystallizer. Related literature reports that this defect is caused by the fluctuation of the crystallizer liquid level, but the improvement effect of implementing measures such as controlling the liquid level fluctuation is not obvious. And the defect disappears after the continuous casting billet is surface-grinded, so it is determined that the defect comes from the surface of the billet.

[0004] The present invention develops a method for solving the black stripe defect on the edge of a stainless steel cold-rolled plate. The black stripe defect on the edge of the cold-rolled plate is solved by rolling the continuous casting billet without grinding, thereby realizing hot-rolling of the continuous casting billet without grinding. While improving product quality, production efficiency and cost are significantly improved. Summary of the invention

[0005] The purpose of the present invention is to provide a method for solving the black stripe defect on the edge of stainless steel cold-rolled plate in view of the above-mentioned problem.

[0006] The object of the present invention is achieved as follows: A method for solving the black streak defect on the edge of a stainless steel cold-rolled plate comprises the following aspects: (1) Design of protective slag: basicity (CaO / SiO 2): 0.7-0.9, melting point: 1100±50℃, C-Total: <2.5%, viscosity: 0.15-0.35dPa.S; (2) After the qualified molten steel produced by the front refining process, the refining end temperature is controlled at 1560-1600℃, ensuring that the superheat of the molten steel in the continuous casting tundish is 20-60℃; (3) The insertion depth of the continuous casting immersion nozzle is controlled at 120-140mm; (4) Before the continuous casting starts, the crystallizer cooling water is set to 100000 square meters of copper plate cooling water according to the size of the crystallizer copper plate area. The flow rate is 2800-3000L / min; (5) The automatic liquid level control system of the continuous casting crystallizer is put into use normally, and the liquid level fluctuation is less than ±5mm; (6) After the continuous casting starts, when the molten steel level of the crystallizer submerges the side holes of the immersion nozzle, the designed protective slag is quickly added to the crystallizer; (7) When the continuous casting speed is increased to the normal casting speed of 0.80-1.30m / min, the protective slag is added stably and evenly to ensure that the slag surface does not show red; (8) The thickness of the protective slag is controlled within the range of 15mm±2mm by fine-tuning the casting speed.

[0007] (3) The inclination angle of the side hole of the mid-immersion nozzle is between 10° and 15° upward.

[0008] The beneficial effects of the present invention are as follows: the present invention improves the stability of continuous casting operation, completely solves the black stripe defect on the edge of stainless steel cold-rolled plate, and at the same time improves the control accuracy of continuous casting billet width, thereby improving the satisfaction of downstream hot rolling, cold rolling and terminal users. DETAILED DESCRIPTION

[0009] The black stripe defect on the edge of cold-rolled stainless steel sheet is a typical defect on the edge of stainless steel with low chromium content, which seriously affects the quality image of the product. After analysis, the main reason for this is that the protective slag that is not completely melted during continuous casting is rolled into the space between the billet shell and the copper plate of the crystallizer, and the C that is not completely burned penetrates into the billet shell of the cast billet and cannot be removed by pickling of the cold-rolled sheet, forming a defect with obvious color difference from the matrix, and it appears black under natural light. After optimizing and improving the C content of the protective slag, the continuous casting flow field, etc., the effect is not obvious. Relevant literature reports that this defect is caused by the high C content of the protective slag, but the effect of conducting experiments by reducing the C content of the protective slag is not obvious. The present invention achieves good results by improving the cooling of the crystallizer and the indicators of the protective slag. The present invention effectively solves this defect by improving the heat transfer of the protective slag and the cooling strength of the crystallizer.

[0010] The present invention solves the problem of C in the mold slag penetrating into the continuous casting shell by optimizing the melting rate, viscosity and other indicators of the mold slag. The process is characterized by good stability and strong process fault tolerance, and the fluctuation of the crystallizer liquid level control, the fluctuation of the superheat of the molten steel, the fluctuation of the seasonal cooling water temperature, the fluctuation of the operation, etc. will not affect the quality.

[0011] The method increases the shell thickness of the ingot by enhancing the heat transfer between the continuous casting cooling water and the protective slag, and reduces the risk of the liquid level fluctuation entraining the incompletely melted protective slag by increasing the thickness of the liquid slag in the crystallizer.

[0012] The key points of the method are: enhancing the continuous casting cold strength, increasing the thickness of the ingot shell and the strength of the shell after leaving the crystallizer, and reducing the liquid level fluctuation caused by roller extrusion; by increasing the melting rate of the protective slag, the thickness of the liquid slag is greater than the amplitude of the crystallizer liquid level fluctuation.

[0013] The design ideas mainly include the following aspects: improving the continuous casting and cooling effect by increasing the flow rate of cooling water in the continuous casting crystallizer and the heat transfer performance of the protective slag, and improving the strength of the billet shell out of the crystallizer; optimizing the melting rate of the protective slag, stabilizing the thickness of the liquid protective slag in the crystallizer, and achieving uniform coverage of the liquid slag, so that the C in the protective slag cannot penetrate into the continuous casting billet shell; optimizing the viscosity of the liquid protective slag to ensure that the liquid slag evenly penetrates into the gap between the billet shell and the copper plate of the crystallizer, achieving uniform cooling of the billet shell, and solving the problem of billet cracks caused by strong cooling during continuous casting and cooling.

[0014] The key control parameters for the implementation of this plan include the following points: 1. The design of protective slag is as shown in the following table.

[0015]

[0016] 2. Continuous casting crystallizer cooling water: When the casting speed is in the range of 0.80-1.30m / min, the cooling water flow rate of the crystallizer copper plate per square meter is 2800-3000L / min.

[0017] The key control parameters for the implementation of this plan include the following: 1. For qualified molten steel produced through the front refining process, the refining endpoint temperature is controlled well to ensure that the superheat of the molten steel in the continuous casting tundish is between 20-60℃. 2. The insertion depth of the continuous casting immersion nozzle is controlled at 120-140mm, and the inclination angle of the immersion nozzle side hole is between 10°-15° upward. 3. Before the continuous casting starts, the cooling water of the crystallizer is set to a flow rate of >2800L / min per square meter of copper plate cooling water according to the size of the copper plate area of ​​the crystallizer (both the wide and narrow sides are set according to this flow rate). 4. The automatic liquid level control system of the continuous casting crystallizer is put into use normally, and the liquid level fluctuation is <±5mm. 5. After the continuous casting starts, when the molten steel level of the crystallizer submerges the side hole of the immersion nozzle, quickly add the designed protective slag to the crystallizer. 6. When the continuous casting pulling speed is increased to the normal casting speed, add protective slag stably and evenly to ensure that the slag surface does not show red. 7. Control the thickness of the protective slag liquid slag within the range of 15mm±2mm by fine-tuning the pulling speed. Implementation Case 1 (410 stainless steel)

[0018] 1. In the front refining process, the molten steel with good purity and composition meeting the composition requirements in Table 2 is adjusted to 1550-1670℃ and then handed over to the continuous casting process.

[0019] 2. For continuous casting, the flow rate of the first cooling water is set to 2800-3000L / min per square meter of copper plate, and the second cooling water is set according to the water volume of 1.3-1.6L / kg.

[0020] 3. For continuous casting, open the baked tundish (refractory material temperature > 1000℃) to the casting position and open the argon gas for purging. After the purging time is > 5 minutes, the long nozzle is put on the ladle outlet through the continuous casting ladle manipulator (a sealing ring is added between the long nozzle bowl and the ladle outlet), and then the ladle slide is opened through the hydraulic system to allow the molten steel to be injected into the continuous casting tundish.

[0021] 4. When the molten steel level in the tundish submerges the lower edge of the long nozzle, start adding the tundish covering agent. The amount of covering agent added ensures that the covering agent can evenly cover the surface of the molten steel.

[0022] 5. When the amount of molten steel in the tundish reaches 80% of the maximum capacity of the tundish, measure the temperature of the molten steel in the tundish. When the temperature meets the range of 1520-1540℃, open the stopper rod of the tundish and start to inject the molten steel in the tundish into the continuous casting crystallizer through the submerged nozzle (integral / split). When the liquid level in the crystallizer submerges the side hole of the submerged nozzle, immediately add the protective slag in Table 3 and start casting until the pulling speed reaches the normal casting speed of 1.1-1.3m / min.

[0023] 6. The ingots are cut according to the specifications required by the plan, the head and tail skins of the continuous casting are ground normally, and the other ingots are directly handed over to the next process for rolling.

[0024] Implementation Case 2 (409L)

[0025] 1. In the front refining process, the molten steel with good purity and composition meeting the composition requirements in Table 4 is adjusted to 1590-1600℃ and then handed over to the continuous casting process.

[0026] 2. For continuous casting, the flow rate of the first cooling water is set to 2800-3000L / min per square meter of copper plate, and the second cooling water is set according to the water volume of 1.3-1.6L / kg.

[0027] 3. For continuous casting, open the baked tundish (refractory material temperature > 1000℃) to the casting position and open the argon gas for purging. After the purging time is > 5 minutes, the long nozzle is put on the ladle outlet through the continuous casting ladle manipulator (a sealing ring is added between the long nozzle bowl and the ladle outlet), and then the ladle slide is opened through the hydraulic system to allow the molten steel to be injected into the continuous casting tundish.

[0028] 4. When the molten steel level in the tundish submerges the lower edge of the long nozzle, start adding the tundish covering agent. The amount of covering agent added ensures that the covering agent can evenly cover the surface of the molten steel.

[0029] 5. When the amount of molten steel in the tundish reaches 80% of the maximum capacity of the tundish, measure the temperature of the molten steel in the tundish. When the temperature meets the range of 1540-1570℃, open the stopper rod of the tundish and start to inject the molten steel in the tundish into the continuous casting crystallizer through the submerged nozzle (integral / split). When the liquid level in the crystallizer submerges the side hole of the submerged nozzle, immediately add the protective slag in Table 5 and start casting until the pulling speed reaches the normal casting speed of 0.90-1.20m / min.

[0030] 6. The ingots are cut according to the specifications required by the plan, the head and tail skins of the continuous casting are ground normally, and the other ingots are directly handed over to the next process for rolling.

[0031]

[0032] The above description is only a specific embodiment of the present invention, but the structural features of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are all covered by the patent scope of the present invention.

Claims

1. A method for solving the black stripe defect on the edge of a stainless steel cold-rolled sheet, characterized in that: These include: (1) Design of mold slag: basicity (CaO / SiO2): 0.7-0.9, melting point: 1100±50℃, C-Total: <2.5%, viscosity: 0.15-0.35dPa.S; (2) After the qualified molten steel produced by the previous refining process, the refining end temperature is controlled at 1560-1600℃, and the superheat of the molten steel in the continuous casting tundish is ensured to be between 20-60℃; (3) The insertion depth of the continuous casting immersion nozzle is controlled at 120-140mm; (4) Before continuous casting begins, set the cooling water flow rate of the crystallizer to 2800-3000 L / min per square meter of copper plate according to the area of ​​the crystallizer copper plate; (5) The automatic liquid level control system of the continuous casting crystallizer is put into use normally, and the liquid level fluctuation is less than ±5mm; (6) After continuous casting starts, when the molten steel level of the crystallizer submerges the side hole of the immersion nozzle, quickly add the designed protective slag into the crystallizer; (7) When the continuous casting speed is increased to the normal casting speed of 0.80-1.30m / min, add protective slag steadily and evenly to ensure that the slag surface does not show red; (8) The thickness of the protective slag liquid is controlled within the range of 15 mm ± 2 mm by fine-tuning the pulling speed.

2. A method for solving the black stripe defect on the edge of a stainless steel cold-rolled sheet according to claim 1, characterized in that: (3) The inclination angle of the side hole of the mid-immersion nozzle is between 10° and 15° upward.

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