A method and system for controlling slag defects on the surface of cold-rolled steel coils

Abstract

This invention discloses a method and system for controlling slag entrapment defects on the surface of cold-rolled steel coils. The method includes: controlling the temperature and composition of molten iron in the pretreatment process based on the steel grade and smelting process; controlling the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade; wherein the oxygen content at the converter endpoint ranges from 350ppm to 500ppm; performing slag modification on the top slag after tapping from the converter; controlling the vacuum degree in the refining process, with oxygen controlled below 350ppm before deoxidation, and the pure circulation time controlled above 6 minutes, followed by secondary slag modification after refining; and in the continuous casting process, using a pre-structured tundish and determining the submerged entry nozzle, nozzle angle, and argon flow rate based on the steel grade to control liquid level fluctuations and reduce slag entrapment.

Description

Technical Field

[0001] This application relates to the field of steelmaking process technology, and in particular to a method and system for controlling slag defects on the surface of cold-rolled steel coils. Background Technology

[0002] With the increasing production of high-surface-grade steel for cold-rolled products, the problem of defect control in continuously cast billets has become increasingly prominent, especially the slag defects on the surface of steel coils, which has become a bottleneck problem in the steelmaking process.

[0003] Research has revealed that the mechanism leading to slag entrapment is complex and influenced by numerous factors. For example, unsteady three-phase turbulent flow and fluctuations within the crystallizer can also cause slag entrapment problems. Currently used Reynolds time-averaged methods cannot fully analyze the turbulent flow patterns and therefore cannot solve the slag entrapment defects on the surface of steel coils.

[0004] Therefore, how to solve the slag defects on the surface of steel coils is a technical problem that urgently needs to be solved. Summary of the Invention

[0005] This invention provides a method and system for controlling slag defects on the surface of cold-rolled steel coils, so as to solve or partially solve the problem of slag defects on the surface of steel coils.

[0006] To address the aforementioned technical problems, a first aspect of the present invention discloses a method for controlling slag defects on the surface of cold-rolled steel coils, the method comprising:

[0007] The temperature and composition of molten iron in the pretreatment process are controlled in conjunction with the steel grade and smelting process.

[0008] The oxygen content and temperature at the converter endpoint in the converter steelmaking process are controlled according to the steel grade; the oxygen content at the converter endpoint ranges from 350ppm to 500ppm.

[0009] After the converter tapps steel, the top slag is modified.

[0010] Control the vacuum level of the refining process, keep the oxygen level below 350 ppm before deoxidation, keep the pure circulation time above 6 minutes, and carry out secondary slag modification after refining.

[0011] In the continuous casting process, a tundish with a pre-designed structure is used, and the submerged entry nozzle, nozzle angle, and argon flow rate are determined in combination with the steel grade, thereby controlling liquid level fluctuations and reducing slag entrapment.

[0012] Preferably, the control of molten iron temperature and composition in the molten iron pretreatment process, combined with steel grade and smelting process, specifically includes:

[0013] If the steel grade is ordinary automotive steel sheet, control the molten iron temperature entering the furnace to be ≥1320℃ and the Si content to be ≥0.10%;

[0014] If the steel grade is bake-hardening steel and conventional smelting is used, control the molten iron temperature at the furnace to be ≥1350℃ and the Si content to be ≥0.15%;

[0015] If the steel grade is bake-hardening steel and a duplex process is used, the molten iron temperature should be controlled at ≥1330℃ and Si ≥0.15%.

[0016] Preferably, the control of the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade specifically includes:

[0017] If the steel grade is ordinary automotive steel sheet, the oxygen control range at the furnace endpoint should be controlled between 360ppm and 480ppm, and the endpoint temperature should be between 1655℃ and 1675℃.

[0018] If the steel is bake-hardening steel, the oxygen control range at the converter endpoint is 380ppm-450ppm, and the endpoint temperature is 1670℃-1690℃.

[0019] Preferably, the slag modification of the top slag after tapping from the converter specifically includes:

[0020] The amount of modifier added should be controlled to be above 1.67 kg / t steel, and the modified slag TFe content should be 3.4% ≤ 4.7% after modification.

[0021] If the total iron content of the top slag after modification is guaranteed to be >4.7%, for every 0.5% increase, the modifier needs to be increased by 0.167 kg / t.

[0022] Preferably, the vacuum degree of the controlled refining process is controlled to be below 350 ppm before deoxidation, the pure circulation time is controlled to be above 6 minutes, and secondary slag upgrading is carried out after refining, specifically including:

[0023] Control the vacuum level of the refining process to <100Pa;

[0024] The oxygen content before deoxidation is controlled at 250ppm-320ppm; the oxygen content before deoxidation and the TFe content in the top slag are balanced by the following formula: , Indicates oxygen before deoxygenation;

[0025] The amount of modifier added for secondary slag upgrading should be controlled above 0.67 kg / t.

[0026] Preferably, in the intermediate package of the preset structure, the two intermediate dams and two retaining walls of the intermediate package are symmetrically distributed with respect to the center point of the intermediate package;

[0027] Among them, the ratio of the distance between two symmetrically distributed intermediate dams to the length of the intermediate moat is controlled at 0.45-0.46; the ratio of the distance between two symmetrically distributed retaining walls to the length of the intermediate moat is controlled at 0.28-0.29.

[0028] The heights of the intermediate dam, retaining wall, and flow stabilizer are proportional to the height of the tundish: the ratio of the height of the intermediate dam to the height of the tundish is 0.30-0.31, the ratio of the height of the retaining wall to the height of the tundish is 0.70-0.69, and the ratio of the height of the flow stabilizer to the height of the tundish is 0.30-0.31.

[0029] Preferably, the determination of the submersible nozzle, nozzle inclination angle, and argon flow rate based on the steel grade specifically includes:

[0030] The immersion depth for ordinary low-carbon steel is 160mm-200mm;

[0031] The immersion depth for ordinary automotive steel sheets is 140mm-180mm;

[0032] The immersion depth of high-grade automotive steel sheets is 180mm.

[0033] Preferably, the determination of the submersible nozzle, nozzle inclination angle, and argon flow rate based on the steel grade specifically includes:

[0034] Different cross-sections use water nozzles with different inclination angles, and the ratio of the cross-sectional area of ​​the water nozzle's inner cavity to the side opening of the water nozzle is controlled between 2.0 and 2.1; among which,

[0035] 900mm≤Cross section≤1600mm, the angle of the immersion nozzle is 45°, and the ratio of the cross-sectional area of ​​the nozzle cavity to the side hole of the nozzle is controlled at 2.0;

[0036] 1600mm < cross-section ≤ 2100mm, immersion nozzle angle is 20°, and the ratio of the cross-sectional area of ​​the nozzle cavity to the side hole of the nozzle is controlled at 2.0.

[0037] Preferably, the determination of the submersible nozzle, nozzle inclination angle, and argon flow rate based on the steel grade specifically includes:

[0038] The flow rates of the three argon gas streams are controlled according to the cross-sectional gradient to ensure that the liquid surface velocity in the crystallizer is controlled within 0.3 m / s; wherein, the three argon gas streams are stopper rod argon gas, upper water inlet argon gas, and plate-to-plate argon gas;

[0039] 900mm≤Cross section≤1300mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 3L / min, 3L / min, and 6L / min, respectively;

[0040] For sections with a cross-section of 1300mm and a diameter of 1600mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 4L / min, 4L / min, and 6L / min, respectively.

[0041] For sections with a cross-section of 1600mm and a diameter of 1800mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 5L / min, 5L / min, and 6L / min, respectively.

[0042] For sections with a cross-section of 1800mm and a diameter of ≤2100mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 7L / min, 7L / min, and 6L / min, respectively.

[0043] A second aspect of the present invention discloses a system for controlling slag defects on the surface of cold-rolled steel coils, comprising:

[0044] The first control unit is used to control the temperature and composition of molten iron in the molten iron pretreatment process in combination with the steel grade and smelting process.

[0045] The second control unit is used to control the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade; wherein, the oxygen content at the converter endpoint ranges from 350ppm to 500ppm.

[0046] The slag modification unit is used to modify the top slag after the converter tapps.

[0047] The third control unit is used to control the vacuum degree of the refining process. The oxygen before deoxidation is controlled within 350 ppm, the pure circulation time is controlled within 6 minutes, and secondary slag upgrading is carried out after refining.

[0048] The continuous casting control unit is used in the continuous casting process to control liquid level fluctuations by using a tundish with a preset structure and determining the submerged entry nozzle, nozzle inclination angle, and argon flow rate in combination with the steel grade, thereby reducing slag entrapment.

[0049] Through one or more technical solutions of the present invention, the present invention has the following beneficial effects or advantages:

[0050] This invention discloses a method and system for controlling slag entrapment defects on the surface of cold-rolled steel coils. The method includes: controlling the temperature and composition of molten iron in the pretreatment process based on the steel grade and smelting process; controlling the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade; wherein the oxygen content at the converter endpoint ranges from 350ppm to 500ppm; performing slag modification on the top slag after tapping from the converter; controlling the vacuum degree in the refining process, with oxygen controlled below 350ppm before deoxidation, and the pure circulation time controlled above 6 minutes; performing secondary slag modification after refining; and in the continuous casting process, using a pre-structured tundish and determining the submerged entry nozzle, nozzle angle, and argon flow rate based on the steel grade to control liquid level fluctuations and reduce slag entrapment. The technical solution of this invention optimizes the entire steelmaking process parameters, especially in the continuous casting process, by optimizing the tundish structure and parameters closely related to the flow field morphology, such as the submerged entry nozzle, nozzle angle, and argon flow rate, thereby reducing the incidence of slag entrapment defects on the surface of cold-rolled coils and improving the cleanliness of the molten steel.

[0051] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0052] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0053] Figure 1 A flowchart of a method for controlling slag defects on the surface of cold-rolled steel coils according to an embodiment of the present invention is shown;

[0054] Figure 2 A schematic diagram of a system for controlling surface slag defects in cold-rolled steel coils according to an embodiment of the present invention is shown. Detailed Implementation

[0055] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0056] This invention discloses a method for controlling slag defects on the surface of cold-rolled steel coils. The method optimizes all process parameters to improve the cleanliness of the molten steel. (See attached document.) Figure 1 The method includes the following steps:

[0057] Step 101: Control the temperature and composition of molten iron in the molten iron pretreatment process in combination with steel grade and smelting process.

[0058] In this embodiment, the temperature and composition of the molten iron must meet the smelting conditions. If the steel grade is ordinary automotive steel sheet, the temperature of the molten iron entering the furnace should be ≥1320℃ and the Si content should be ≥0.10%. If the steel grade is bake-hardening steel (high-grade automotive steel sheet) and conventional smelting is used, the temperature of the molten iron entering the furnace should be ≥1350℃ and the Si content should be ≥0.15%. If the steel grade is bake-hardening steel and a duplex process is used, the temperature of the molten iron should be ≥1330℃ and the Si content should be ≥0.15%.

[0059] Step 102: Control the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade.

[0060] The final oxygen content range for the converter is 350ppm-500ppm. Specifically, different steel grades have different final oxygen contents and temperatures. For ordinary automotive steel, the final oxygen content is controlled between 360ppm and 480ppm, with a target of 420ppm, and a final temperature of 1655℃-1675℃. For bake-hardening steel, the final oxygen content is controlled between 380ppm and 450ppm, and the final temperature is 1670℃-1690℃.

[0061] Step 103: After tapping steel from the converter, the top slag is modified.

[0062] During the slag modification process, the amount of modifier added should be controlled to be above 1.67 kg / t steel, and the top slag TFe content after modification should be 3.4% ≤ 4.7%; if the total iron content of the top slag after modification is guaranteed to be > 4.7%, the modifier should be increased by 0.167 kg / t for every 0.5% increase.

[0063] Step 104: Control the vacuum level of the refining process, keep the oxygen level below 350 ppm before deoxidation, keep the pure circulation time above 6 minutes, and carry out secondary slag modification after refining.

[0064] In this embodiment, the vacuum level of the refining process is controlled to be <100 Pa. The amount of oxygen before deoxidation needs to be balanced with the TFe in the modified top slag. The balance between oxygen before deoxidation and TFe in the top slag is as follows: , This refers to the oxygen content before deoxidation. For example, the oxygen content before deoxidation can be controlled at 250ppm-320ppm, corresponding to 3.4%≤Top Slag TFe≤4.7% after modification;

[0065] In this embodiment, the pure cycle time must reach more than 6 minutes. Furthermore, the pure cycle time is limited to the cycle time after adding the aluminum alloy, and the time interval between adding the titanium alloy and the aluminum alloy is required to be more than 3 minutes.

[0066] In this embodiment, the amount of modifier added for secondary slag modification is controlled to be above 0.67 kg / t. For example, 0.67 kg / t of modifier is added to the slag surface of the ladle.

[0067] Step 105: In the continuous casting process, a tundish with a pre-designed structure is used, and the submerged entry nozzle, nozzle angle, and argon flow rate are determined in conjunction with the steel grade, thereby controlling liquid level fluctuations and reducing slag entrapment.

[0068] The unsteady three-phase turbulent flow and fluctuations of gas, liquid, and slag within the crystallizer are the root cause of slag entrapment. The tundish, located between the ladle and the crystallizer and the last container before the casting process begins, not only serves as a transitional container for molten steel, but its structure also directly impacts the quality of steelmaking refining. Furthermore, the continuous casting cross-section, argon flow rate, submerged entry nozzle angle, and immersion depth are closely related to the flow field morphology. Therefore, this embodiment comprehensively considers the above factors in the continuous casting process to improve the flow field in the tundish and crystallizer, thereby reducing slag entrapment and improving the cleanliness of the molten steel.

[0069] Specifically, in this embodiment, a pre-defined intermediate package structure is used, and the structure of the intermediate package is described below. The two central dams of the intermediate package are symmetrically distributed with respect to the center point of the intermediate package; similarly, the two retaining walls of the intermediate package are also symmetrically distributed with respect to the center point of the intermediate package.

[0070] Among them, the distance between two symmetrically distributed intermediate dams is proportional to the length of the intermediate tundish, with the ratio controlled between 0.45 and 0.46; the distance between two symmetrically distributed retaining walls is proportional to the length of the intermediate tundish, with the ratio controlled between 0.28 and 0.29.

[0071] In addition, the heights of the intermediate dam, retaining wall, and flow stabilizer are proportional to the height of the tundish: the ratio of the height of the intermediate dam to the height of the tundish is 0.30-0.31, the ratio of the height of the retaining wall to the height of the tundish is 0.70-0.69, and the ratio of the height of the flow stabilizer to the height of the tundish is 0.30-0.31.

[0072] In this embodiment, the structural dimensions of the tundish are set according to a proportional relationship. The purpose is to fully extend the residence time of the molten steel in the tundish, reduce the volume fraction of the dead zone, increase the volume fraction of the piston zone, extend the time for adjusting the temperature and composition of the molten steel and removing inclusions, and obtain the ideal flow state of the molten steel through precise flow field control to reduce heat loss and refractory corrosion.

[0073] In this embodiment, the immersion gate is selected according to the steel grade. Specifically, the immersion depth is 160mm-200mm for ordinary low-carbon steel, 140mm-180mm for ordinary automotive steel, and 180mm for high-grade automotive steel.

[0074] In this embodiment, different inclination angles are used for different cross-sections of the sprue, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled between 2.0 and 2.1. The larger the sprue outlet area, the weaker the flow stream. However, with a large sprue outlet area, the jet impact angle of the outlet flow stream is not easily controlled by the designed inclination angle, and the inclination angle of the main flow outlet is generally larger than that of the designed sprue.

[0075] Specifically, the selection of the inclination angle and cross-sectional area ratio of the submersible nozzle is as follows:

[0076] 900mm≤Cross section≤1600mm, the angle of the submerged nozzle is 45° to ensure that the direction of the molten steel flow is downward, reducing the formation of backflow. The ratio of the cross-sectional area of ​​the nozzle cavity to the side hole of the nozzle is controlled at 2.0 to reduce the scouring of the narrow solidified billet shell by the flow.

[0077] For sections with a cross-section of 1600mm to 2100mm, the angle of the submerged nozzle is 20°. The ratio of the cross-sectional area of ​​the nozzle cavity to the cross-sectional area of ​​the nozzle side hole is controlled at 2.0. Large cross-sections adopt small angles and large cross-sectional area ratios to increase the flow of molten steel towards the narrow face, activate the flow field of the entire cross-section, and facilitate the uniformity of the inflow of protective slag.

[0078] In this embodiment, the flow rates of the three argon gas streams are controlled according to the cross-sectional gradient to ensure that the liquid surface velocity in the crystallizer is controlled within 0.3 m / s; wherein, the three argon gas streams are stopper rod argon gas, upper water inlet argon gas, and plate-to-plate argon gas;

[0079] 900mm≤Cross section≤1300mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 3L / min, 3L / min, and 6L / min, respectively;

[0080] For sections with a cross-section of 1300mm and a diameter of 1600mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 4L / min, 4L / min, and 6L / min, respectively.

[0081] For sections with a cross-section of 1600mm and a diameter of 1800mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 5L / min, 5L / min, and 6L / min, respectively.

[0082] For sections with a cross-section of 1800mm and a diameter of ≤2100mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the interplate argon are controlled at 7L / min, 7L / min, and 6L / min, respectively.

[0083] To illustrate and explain the present invention, specific examples are provided below.

[0084] Example 1

[0085] Heat batch 1, production cross section 1300mm, producing ordinary automotive steel, molten iron temperature 1350℃, Si content 0.2%, converter endpoint oxygen content 400ppm, endpoint temperature 1660℃, modifier addition 1.67kg / t steel, top slag total iron 3.5%, refining vacuum 90Pa, oxygen before deoxidation 310ppm, pure circulation time 6min, secondary modifier addition 0.167kg / t steel, the ratio of the distance between the two dams in the tundish to the tundish length is 0.45, the ratio of the distance between the two retaining walls to the tundish length is 0.28, the ratio of the dam height to the tundish height is 0.30, the ratio of the retaining wall height to the tundish height is 0.70, the ratio of the flow stabilizer height to the tundish height is 0.30, the nozzle angle is 45°, the immersion depth is 140-180mm, and the three-way argon flow rate is 3 / 3 / 6L / min.

[0086] Example 2

[0087] Heater 2, production cross-section 2000mm, producing high-grade automotive steel, molten iron temperature 1340℃, Si content 0.23%, converter final oxygen content 360ppm, final temperature 1680℃, modifier addition 1.67kg / t steel, top slag total iron 5%, supplementary modifier 0.334kg / t, steel refining vacuum 75Pa, oxygen before deoxidation 280ppm, pure circulation time 7min, secondary modifier addition 0.167kg / t steel, the ratio of the distance between the two dams in the tundish to the tundish length is 0.46, the ratio of the distance between the two retaining walls to the tundish length is 0.29, the ratio of the dam height to the tundish height is 0.31, the ratio of the retaining wall height to the tundish height is 0.69, the ratio of the flow stabilizer height to the tundish height is 0.31, the nozzle angle is 20°, the immersion depth is 200mm, and the three-way argon flow rate is 7 / 7 / 6L / min.

[0088] Comparative Example 1

[0089] Heat batch 1, production cross section 1300mm, producing ordinary automotive steel, molten iron temperature 1330℃, Si content 0.25%, converter endpoint oxygen content 410ppm, endpoint temperature 1660℃, modifier addition 1.67kg / t steel, top slag total iron 3.5%, refining vacuum 90Pa, oxygen before deoxidation 310ppm, pure circulation time 6min, secondary modifier addition 0.167kg / t steel, the ratio of the distance between the two dams in the tundish to the tundish length is 0.42, the ratio of the distance between the two retaining walls to the tundish length is 0.26, the ratio of the dam height to the tundish height is 0.28, the ratio of the retaining wall height to the tundish height is 0.72, the ratio of the flow stabilizer height to the tundish height is 0.28, the nozzle angle is 45°, the immersion depth is 140-180mm, and the three-way argon flow rate is 3 / 3 / 6L / min.

[0090] Comparative Example 2

[0091] Heat batch 2, production cross-section 2100mm, producing high-grade automotive steel, molten iron temperature 1350℃, Si content 0.23%, converter endpoint oxygen content 360ppm, endpoint temperature 1680℃, modifier addition 1.67kg / t steel, top slag total iron 5%, supplementary modifier 0.334kg / t, steel refining vacuum degree 90Pa, oxygen before deoxidation 280ppm, pure circulation time 6.5min, secondary modifier addition. The feed rate is 0.167 kg / t steel. The ratio of the distance between the two dams to the length of the tundish is 0.48. The ratio of the distance between the two retaining walls to the length of the tundish is 0.30. The ratio of the dam height to the tundish height is 0.33. The ratio of the retaining wall height to the tundish height is 0.66. The ratio of the flow stabilizer height to the tundish height is 0.33. The nozzle angle is 20°. The immersion depth is 200 mm. The three-way argon flow rate is 7 / 7 / 6 L / min.

[0092] The effects of the examples and comparative examples are shown in Table 1.

[0093] Table 1

[0094]

[0095] The technical solution of this invention optimizes the process parameters of the entire steelmaking process. In particular, in the continuous casting process, it optimizes the structure of the tundish and parameters closely related to the flow field morphology, such as the submerged entry nozzle, the nozzle inclination angle, and the argon flow rate, thereby reducing the incidence of slag defects on the surface of cold-rolled coils and improving the cleanliness of the molten steel.

[0096] Based on the same inventive concept as one or more of the foregoing embodiments, the following embodiments disclose a system for controlling slag defects on the surface of cold-rolled steel coils, see below. Figure 2 ,include:

[0097] The first control unit 201 is used to control the temperature and composition of molten iron in the molten iron pretreatment process in combination with the steel grade and smelting process.

[0098] The second control unit 202 is used to control the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade; wherein, the oxygen content at the converter endpoint ranges from 350ppm to 500ppm.

[0099] Slag upgrading unit 203 is used to upgrade the top slag after the converter tapps.

[0100] The third control unit 204 is used to control the vacuum degree of the refining process, control the oxygen before deoxidation to within 350ppm, control the pure circulation time to more than 6min, and carry out secondary slag upgrading after refining.

[0101] The continuous casting control unit 205 is used in the continuous casting process to use a tundish with a preset structure and to determine the submerged entry nozzle, nozzle angle and argon flow rate in combination with the steel grade, thereby controlling liquid level fluctuations and reducing slag entrapment.

[0102] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0103] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for controlling slag defects on the surface of cold-rolled steel coils, characterized in that, The method includes: The temperature and composition of molten iron in the pretreatment process are controlled in conjunction with the steel grade and smelting process. The oxygen content and temperature at the converter endpoint in the converter steelmaking process are controlled according to the steel grade; the oxygen content at the converter endpoint ranges from 350ppm to 500ppm. After the converter tapps steel, the top slag is modified. Control the vacuum level of the refining process, keep the oxygen level below 350 ppm before deoxidation, keep the pure circulation time above 6 minutes, and carry out secondary slag modification after refining. In the continuous casting process, a tundish with a pre-designed structure is used, and the submerged entry nozzle, nozzle angle, and argon flow rate are determined in conjunction with the steel grade to control liquid level fluctuations and reduce slag entrapment. In the pre-designed tundish, two central dams and two retaining walls are symmetrically distributed with respect to the tundish center point. The ratio of the distance between the two symmetrically distributed central dams to the tundish length is controlled between 0.45 and 0.46; the ratio of the distance between the two symmetrically distributed retaining walls to the tundish length is controlled between 0.28 and 0.

29. The heights of the central dams, retaining walls, and flow stabilizers are proportional to the height of the tundish: the ratio of the central dam height to the tundish height is 0.30-0.31, the ratio of the retaining wall height to the tundish height is 0.70-0.69, and the ratio of the flow stabilizer height to the tundish height is 0.30-0.

31. The immersion depth for ordinary low-carbon steel is 160mm-200mm; the immersion depth for ordinary automotive steel is 140mm-180mm; and the immersion depth for high-grade automotive steel is 180mm. Different inclination angles are used for different cross-sections of the sprue, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled between 2.0 and 2.1; among them, for cross-sections of 900mm ≤ 1600mm, the angle of the submersible sprue is 45°, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled at 2.0; for cross-sections of 1600mm < 2100mm, the angle of the submersible sprue is 20°, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled at 2.

0. The flow rates of the three argon gas streams are controlled in a gradient manner according to the cross-section to ensure that the liquid surface velocity in the crystallizer is controlled within 0.3 m / s. The three argon gas streams are: stopper rod argon, upper inlet argon, and inter-plate argon. For cross-sections of 900 mm ≤ cross-section ≤ 1300 mm, the flow rates of the stopper rod argon, upper inlet argon, and inter-plate argon are controlled at 3 L / min, 3 L / min, and 6 L / min, respectively. For cross-sections of 1300 mm < cross-section ≤ 1600 mm, the flow rates of the stopper rod argon, upper inlet argon, and inter-plate argon... The flow rates of the gas are controlled at 4L / min, 4L / min, and 6L / min, respectively; for sections 1600mm < cross-section ≤ 1800mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the inter-plate argon are controlled at 5L / min, 5L / min, and 6L / min, respectively; for sections 1800mm < cross-section ≤ 2100mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the inter-plate argon are controlled at 7L / min, 7L / min, and 6L / min, respectively.

2. The method as described in claim 1, characterized in that, The control of molten iron temperature and composition in the molten iron pretreatment process, which combines steel grade and smelting process, specifically includes: If the steel grade is ordinary automotive steel sheet, control the molten iron temperature entering the furnace to be ≥1320℃ and the Si content to be ≥0.10%; If the steel grade is bake-hardening steel and conventional smelting is used, control the molten iron temperature at the furnace to be ≥1350℃ and the Si content to be ≥0.15%; If the steel grade is bake-hardening steel and a duplex process is used, the molten iron temperature should be controlled at ≥1330℃ and Si ≥0.15%.

3. The method as described in claim 1, characterized in that, The control of the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade specifically includes: If the steel grade is ordinary automotive steel, the oxygen control range at the converter endpoint should be controlled at 360ppm-480ppm, and the endpoint temperature should be 1655℃-1675℃. If the steel is bake-hardening steel, the oxygen control range at the converter endpoint is 380ppm-450ppm, and the endpoint temperature is 1670℃-1690℃.

4. The method as described in claim 1, characterized in that, The process of upgrading the top slag after tapping from the converter specifically includes: The amount of modifier added should be controlled to be above 1.67 kg / t steel, and the modified slag TFe content should be 3.4% ≤ 4.7% after modification.

5. The method as described in claim 1 or 4, characterized in that, The vacuum degree of the controlled refining process is maintained below 350 ppm before deoxidation, the pure circulation time is controlled above 6 minutes, and secondary slag modification is performed after refining, specifically including: Control the vacuum level of the refining process to <100Pa; The oxygen content before deoxidation is controlled at 250ppm-320ppm; the oxygen content before deoxidation and the TFe content in the top slag are balanced by the following formula: , Indicates oxygen before deoxygenation; The amount of modifier added for secondary slag upgrading should be controlled above 0.67 kg / t.

6. A system for controlling slag defects on the surface of cold-rolled steel coils, characterized in that, include: The first control unit is used to control the temperature and composition of molten iron in the molten iron pretreatment process in combination with the steel grade and smelting process. The second control unit is used to control the oxygen content and temperature at the converter endpoint in the converter steelmaking process according to the steel grade; wherein, the oxygen content at the converter endpoint ranges from 350ppm to 500ppm. The slag modification unit is used to modify the top slag after the converter tapps. The third control unit is used to control the vacuum degree of the refining process. The oxygen before deoxidation is controlled within 350 ppm, the pure circulation time is controlled within 6 minutes, and secondary slag upgrading is carried out after refining. A continuous casting control unit is used in the continuous casting process to control liquid level fluctuations by employing a tundish with a preset structure and determining the submerged entry nozzle, nozzle angle, and argon flow rate in conjunction with the steel grade, thereby reducing slag entrapment. In the preset structure tundish, two central dams and two retaining walls are symmetrically distributed with respect to the tundish center point. The ratio of the distance between the two symmetrically distributed central dams to the tundish length is controlled between 0.45 and 0.46; the ratio of the distance between the two symmetrically distributed retaining walls to the tundish length is controlled between 0.28 and 0.29; the heights of the central dams, retaining walls, and flow stabilizers are proportional to the height of the tundish: the ratio of the central dam height to the tundish height is 0.30-0.31, the ratio of the retaining wall height to the tundish height is 0.70-0.69, and the ratio of the flow stabilizer height to the tundish height is 0.30-0.

31. The immersion depth for ordinary low-carbon steel is 160mm-200mm; the immersion depth for ordinary automotive steel is 140mm-180mm; and the immersion depth for high-grade automotive steel is 180mm. Different inclination angles are used for different cross-sections of the sprue, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled between 2.0 and 2.1; among them, for cross-sections of 900mm ≤ 1600mm, the angle of the submersible sprue is 45°, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled at 2.0; for cross-sections of 1600mm < 2100mm, the angle of the submersible sprue is 20°, and the ratio of the cross-sectional area of ​​the sprue cavity to the sprue side hole is controlled at 2.

0. The flow rates of the three argon gas streams are controlled in a gradient manner according to the cross-section to ensure that the liquid surface velocity in the crystallizer is controlled within 0.3 m / s. The three argon gas streams are: stopper rod argon, upper inlet argon, and inter-plate argon. For cross-sections of 900 mm ≤ cross-section ≤ 1300 mm, the flow rates of the stopper rod argon, upper inlet argon, and inter-plate argon are controlled at 3 L / min, 3 L / min, and 6 L / min, respectively. For cross-sections of 1300 mm < cross-section ≤ 1600 mm, the flow rates of the stopper rod argon, upper inlet argon, and inter-plate argon... The flow rates of the gas are controlled at 4L / min, 4L / min, and 6L / min, respectively; for sections 1600mm < cross-section ≤ 1800mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the inter-plate argon are controlled at 5L / min, 5L / min, and 6L / min, respectively; for sections 1800mm < cross-section ≤ 2100mm, the flow rates of the stopper rod argon, the upper water inlet argon, and the inter-plate argon are controlled at 7L / min, 7L / min, and 6L / min, respectively.

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