A method for improving the shape of an if steel cold rolled sheet

Abstract

The application relates to a method for improving the shape of an IF steel cold-rolled plate and belongs to the technical field of steelmaking processes. The method comprises the following steps: heating a casting blank in a furnace and controlling the gap between adjacent casting blanks; wherein the heating in the furnace comprises a preheating section, a first heating section, a second heating section and a soaking section; performing edge heating on the heated casting blank to ensure that the target area of the casting blank is raised to a set temperature; sequentially performing finish rolling and ultra-fast cooling on the casting blank after the edge heating to obtain a hot-rolled coil; wherein the work roll for the finish rolling is subjected to step cooling; and performing cold rolling on the hot-rolled coil to obtain a cold-rolled plate. Through optimization of the hot-rolling process and equipment, the edge structure of the hot-rolled coil is effectively improved, so that the shape defects of the IF steel cold-rolled plate are improved.

Description

Technical Field

[0001] This application relates to the field of steelmaking process technology, and in particular to a method for improving the shape of IF steel cold-rolled sheet. Background Technology

[0002] Interstitial steel (IF steel), also known as interstitial steel, is widely used in numerous fields such as automotive manufacturing, home appliances, and construction due to its excellent deep-drawing performance and superior surface quality. Its unique microstructure and excellent mechanical properties enable IF steel to meet a range of requirements for high strength, high precision, and high corrosion resistance. However, controlling its sheet shape has always been a challenge in the production process, directly affecting product yield and quality. With the manufacturing industry's ever-increasing demands for material performance, exploring and implementing effective sheet shape improvement methods is particularly urgent.

[0003] IF steel cold-rolled coils commonly suffer from edge waviness defects, a type of sheet shape problem that is difficult to solve in the industry. This shape problem is primarily characterized by double-sided waviness; after heat treatment, the waviness degradation rate of galvanized sheets can reach as high as 5%, failing to meet the needs of high-end customers. To improve the sheet shape defects of cold-rolled sheets, the industry typically attempts to adjust cold-rolling process parameters. Patent CN103801580A proposes controlling the final rolling temperature to ensure the absence of mixed grains and deformation structures at the edges, and cold-rolling edge trimming of 15-30mm to ensure the sheet shape, but does not propose specific measures to improve the edge microstructure. Patent CN114226453A solves the waviness problem in ferritic rolling processes by controlling heating temperature, rolling temperature, rolling speed, and reduction rate; however, this patent is specific to ferritic rolling processes and has no reference value for traditional austenitic rolling processes. Patent CN115415332A proposes high-temperature furnace exit temperature and post-hot rolling leveling to improve the sheet shape problem of high-strength steel.

[0004] However, while these methods can alleviate the sheet shape problem to some extent, they cannot fundamentally solve the waviness defects caused by uneven microstructure during hot rolling after cold rolling. During hot rolling, due to the influence of various factors such as rolling pressure, temperature control, and roll condition, uneven internal stress and microstructure changes may occur inside the sheet. These are amplified during cold rolling, ultimately leading to sheet shape defects such as edge waviness. Summary of the Invention

[0005] This application provides a method for improving the shape of IF steel cold-rolled sheets to solve the following technical problem: how to improve the edge wavy shape defect of IF steel cold-rolled sheets through hot rolling process.

[0006] This application provides a method for improving the shape of cold-rolled IF steel sheet, the method comprising:

[0007] The billets are heated in a furnace, and the gap between adjacent billets is controlled; wherein the furnace heating includes: a preheating section, a first heating section, a second heating section, and a soaking section;

[0008] The heated billet is then subjected to edge heating to ensure that the target area of ​​the billet reaches the set temperature.

[0009] The heated billet is then subjected to precision rolling and ultra-rapid cooling to obtain a hot-rolled coil; wherein the work rolls of the precision rolling are subjected to stepped cooling.

[0010] The hot-rolled coil is cold-rolled to obtain a cold-rolled sheet.

[0011] Optionally, the temperature of the heat spreader is 1170℃~1190℃.

[0012] Optionally, the gap between adjacent castings is 200mm to 280mm.

[0013] Optionally, the temperature of the heat exchange section is more than 20°C higher than the temperature of the second heat exchange section.

[0014] Optionally, the target area is within 80mm from the edge.

[0015] Optionally, the set temperature is 50℃~80℃.

[0016] Optionally, the edge roller of the working roller is the area within 200mm of the edge of the working roller. Through the stepped cooling, the temperature of the edge roller is 15℃ to 20℃ lower than the temperature of the middle roller of the working roller.

[0017] Optionally, the reduction rate of the finishing mill stand F6 is 27% to 29%, and the reduction rate of the finishing mill stand F7 is 17% to 19%.

[0018] Optionally, the pressure of the ultra-rapid cooling is 0.45 MPa to 0.55 MPa.

[0019] Optionally, the cooling rate of the ultra-fast cooling is 70℃ / s to 100℃ / s.

[0020] The technical solutions provided in this application have the following advantages compared with the prior art:

[0021] This application provides a method for improving the shape of IF steel cold-rolled sheet. The method includes: heating a billet in a furnace and controlling the gap between adjacent billets; wherein the furnace heating includes a preheating section, a first heating section, a second heating section, and a soaking section; edge heating the heated billet to ensure that the target area of ​​the billet reaches a set temperature; sequentially finishing rolling and ultra-rapid cooling the edge-heated billet to obtain a hot-rolled coil; wherein the finishing rolling work rolls are subjected to stepped cooling; and the hot-rolled coil is cold-rolled to obtain a cold-rolled sheet. By precisely controlling parameters such as temperature during the furnace heating process, the billet temperature is ensured to be uniform, reducing internal stress. Secondly, edge heating can improve the heating uniformity of the strip edge, effectively reducing edge cracks and local deformation, thereby improving the sheet shape quality. Stepped cooling of the finishing rolling work rolls can ensure the working stability of the rolls and reduce sheet shape defects caused by roll thermal deformation. In addition, the optimized configuration of the finishing rolling load can adjust the deformation distribution of various parts of the strip, making it more uniform and reasonable, thereby effectively preventing the occurrence of various sheet shape problems. Finally, the introduction of ultra-rapid cooling can significantly improve the cooling efficiency of strip steel and ensure the uniformity of the microstructure of strip steel during rapid cooling, which has a significant effect on improving the shape of IF steel cold-rolled sheet. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 A flowchart illustrating a method for improving the shape of cold-rolled IF steel sheet provided in this application embodiment;

[0025] Figure 2 Edge microstructure of hot-rolled coils provided as comparative examples in this application;

[0026] Figure 3 The edge structure of the hot-rolled coil provided in the embodiments of this application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range such as 1, 2, 3, 4, 5, and 6, regardless of the range; in addition, whenever a numerical range is indicated herein, it means including any referenced number (fraction or integer) within the indicated range.

[0029] In this document, terms such as “comprising” mean “including but not limited to”. Relational terms such as “first” and “second” are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. “And / or” describes the relationship between related objects, indicating that there can be three relationships, for example, A and / or B can mean: A exists alone, A and B exist simultaneously, or B exists alone; where A and B can be singular or plural. “At least one” means one or more, “more” means two or more; “at least one,” “at least one of the following,” or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, “at least one of a, b, or c,” or “at least one of a, b, and c,” can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts representation" such as parts by weight or parts by mass indicates the proportional relationship between components. In the proportional relationships discussed in this article, the parameters that need to be described by proportion should be understood as the first term of the proportion in the order of description, and the proportion figures should be understood as the second term of the proportion. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substances A, B, and C should correspond one-to-one with the proportion figures in the proportion in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0030] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0031] Figure 1 This is a flowchart illustrating a method for improving the shape of IF steel cold-rolled sheet, provided in an embodiment of this application.

[0032] like Figure 1 As shown in the embodiment of this application, a method for improving the shape of IF steel cold-rolled sheet is provided, the method comprising:

[0033] S1. The billet is heated in the furnace, and the gap between adjacent billets is controlled; wherein the furnace heating includes: a preheating section, a first heating section, a second heating section, and a soaking section;

[0034] First, the cast billets are fed into a heating furnace for heating treatment. This process is divided into a preheating section, a first heating section, a second heating section, and a soaking section. During the heating process, special attention is paid to controlling the gap between adjacent cast billets to ensure heating uniformity and efficiency.

[0035] In the embodiments of this application, the chemical composition of the billet, by mass fraction, includes: C≤0.06%, Si≤0.03%, 0.05%≤Mn≤0.25%, P≤0.017%, S≤0.015%, 0.035%≤Ti≤0.07%, and the matrix element Fe.

[0036] In interstitial steel, a low carbon content is crucial because it helps reduce interstitial atoms (such as carbon and nitrogen), thereby improving the steel's toughness and deep-drawing properties. Silicon is commonly used for deoxidation and as a solid solution strengthening element, but in IF steel, its content needs to be strictly controlled to maintain the steel's purity and properties. Manganese is a common alloying element in steel, used to increase its strength and hardness, while also contributing to deoxidation and desulfurization. In IF steel, the manganese content needs to be properly balanced to ensure good deep-drawing properties and strength. While phosphorus can increase the strength of steel, excessive phosphorus can lead to increased cold brittleness, so its content needs to be strictly controlled in IF steel. Sulfur is one of the main factors contributing to hot brittleness in steel, so its content needs to be strictly controlled in IF steel to ensure good weldability and mechanical properties. Titanium plays a vital role in interstitial steel; it can combine with carbon and nitrogen in the steel to form stable compounds, thereby reducing the number of interstitial atoms and improving the steel's toughness and deep-drawing properties.

[0037] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is:

[0038] The sum of the percentages of all components in a composition shall be equal to 100%, and the content ranges of several components shall meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100.

[0039] In this embodiment of the application, the billet is heated through a preheating section, a first heating section, a second heating section, and a soaking section, and then exits the furnace at the soaking section temperature.

[0040] From the moment the billet enters the furnace, the preheating stage is the initial stage of the billet heating process and a fundamental step in the entire heating process. Its main purpose is to slowly raise the billet's temperature, allowing it to gradually adapt to the ambient temperature inside the furnace, thereby reducing thermal stress caused by excessive temperature differences and preparing it for subsequent heating processes. In this embodiment, the preheating temperature is controlled between 680℃ and 790℃ to avoid changes in microstructure and a decrease in mechanical properties caused by rapid heating.

[0041] Following the preheating section is the heating section, where the billet has adapted to the furnace environment and the heating rate can be accelerated. During this stage, the furnace temperature gradually rises to a high level to achieve deep firing of the billet. In this embodiment, the heating section includes a first heating section and a second heating section. After entering the first heating section, the task is to continue raising the billet temperature and ensure a moderate and uniform heating rate; the temperature of the first heating section is controlled between 1030℃ and 1090℃. Entering the second heating section, the billet continues to be subjected to high temperatures to complete the necessary phase transformation and grain growth processes; the temperature of the second heating section is controlled between 1130℃ and 1170℃.

[0042] The soaking stage is the final stage in the billet heating process and a crucial step in ensuring billet quality. During this stage, precise furnace temperature control is required to ensure the billet is heated thoroughly and evenly under constant or slowly varying temperature conditions. The purpose of the soaking stage is to eliminate potential localized overheating or undercooling, ensuring that temperature differences across the entire billet cross-section are within acceptable limits. This reduces internal stress caused by temperature variations, thereby guaranteeing stable product performance and quality in subsequent processes.

[0043] In some embodiments, the temperature of the heat exchange zone is 1170°C to 1190°C.

[0044] By optimizing the temperature of the soaking zone, the deep-drawing performance of IF steel is ensured, and the lateral temperature difference at low temperatures is smaller on the rolling line, which is beneficial to the uniformity of the edge microstructure. For example, the temperature of the soaking zone can be 1170℃, 1175℃, 1180℃, 1185℃, 1190℃, etc.

[0045] In some embodiments, the temperature of the heat exchange section is more than 20°C higher than the temperature of the secondary heat exchange section.

[0046] This setting ensures that the gas flow rate in the homogenization section can be increased, fully utilizing the heating capacity of the homogenization section to maintain the temperature of the slab surface and edges. For example, the temperature of the homogenization section can be 21°C, 22°C, 23°C, 24°C, 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, etc., higher than the temperature of the secondary heating section.

[0047] In some embodiments, the gap between adjacent castings is 200mm to 280mm.

[0048] Inside the heating furnace, the gap between adjacent billets has a significant impact on the overall heating effect. A gap that is too small may cause the billets to come into contact with each other and become compressed, hindering effective heat conduction and convection, thus affecting the uniformity of billet heating. Conversely, a gap that is too large may cause excessive heat consumption within the heating furnace, reducing energy efficiency and increasing production costs. In this embodiment, the gap between adjacent billets is set to 200mm to 280mm. For example, the gap between adjacent billets can be 200mm, 220mm, 240mm, 260mm, 280mm, etc.

[0049] S2. Heat the edges of the heated billet to ensure that the target area of ​​the billet reaches the set temperature.

[0050] After initial heating, specific target areas (usually the edges) of the billet are further heated to a predetermined temperature. This step aims to improve the heating uniformity of the strip edges, effectively reducing edge cracks and localized deformation caused by uneven temperature, thereby improving the final strip shape quality.

[0051] Edge heating, by applying appropriate amounts of heat to the edges of the billet, can effectively reduce the temperature difference between the edge and the center, thereby reducing plate shape defects such as edge thinning and waviness caused by temperature differences. Edge heating of the preheated billet aims to improve the temperature distribution of the billet through localized heating, thus affecting its shape and quality after rolling.

[0052] In some implementations, the target area is within 80 mm of the edge.

[0053] In some embodiments, the set temperature is 50°C to 80°C.

[0054] In this embodiment, an edge heater is used for edge heating, which effectively solves the problem of low edge temperature in hot-rolled strip. By precisely setting the edge heating temperature and range, the temperature of the strip edge can be made consistent with the center, thereby reducing strip shape defects such as edge waviness caused by temperature differences. Setting the temperature within 80mm of the edge to be 50°C to 80°C higher than the center temperature aims to compensate for the temperature drop at the edge of the rolling line, ensure temperature consistency between the edge and center, and thus improve the edge microstructure of the hot-rolled strip.

[0055] S3. The heated billet is sequentially subjected to precision rolling and ultra-rapid cooling to obtain a hot-rolled coil; wherein the work rolls of the precision rolling are subjected to stepped cooling.

[0056] In the finishing rolling stage, the cooling effect of the work rolls plays a crucial role in the strip shape quality. To achieve efficient and precise shape control, stepped cooling technology is an advanced and effective method. The core of this technology lies in dividing the surface of the work rolls into multiple cooling zones along the axial or radial direction according to the different heat load requirements during operation, and designing and implementing differentiated cooling intensities for each zone. This ensures that the work rolls maintain a uniform temperature distribution during rolling, effectively reducing thermal deformation caused by heat concentration, and thus significantly improving the flatness and overall quality of the strip.

[0057] In some embodiments, the edge roller of the working roller is the area within 200mm of the edge of the working roller, and the temperature of the edge roller is 15°C to 20°C lower than that of the middle roller of the working roller through the stepped cooling.

[0058] Increasing the temperature of the edge rolls can influence the deformation behavior of the rolled piece during rolling, helping to reduce or eliminate sheet shape defects such as edge thinning and waviness, thereby improving the sheet shape quality of the product. In this embodiment, the water flow rate of the three rows of nozzles on the edge of the work roll is adjusted to 40m³ / h. 3 / h, the water flow rate of the nozzle in the middle of the working roller is 85m³ / h. 3 This method, using a speed of [h], results in the edge roll temperature being 15°C to 20°C lower than the center roll temperature of the work roll, thereby lowering the edge temperature of the strip by 20°C to 25°C compared to the center temperature. This increases the temperature difference between the edge and center of the strip by 4°C to 5°C compared to conventional methods. For example, the edge roll temperature can be 15°C, 16°C, 17°C, 18°C, 19°C, or 20°C lower than the center roll temperature of the work roll.

[0059] In some embodiments, the reduction rate of the finishing mill stand F6 is 27% to 29%, and the reduction rate of the finishing mill stand F7 is 17% to 19%.

[0060] In the finishing rolling stage, by cleverly adjusting the load distribution and shifting part of the load to the rear stands, the reduction burden on the front stands can be effectively reduced, thereby decreasing the internal stress caused by excessive reduction. Furthermore, the rear stands have greater deformation resistance; increasing the reduction rate can increase the forward and backward slippage of the strip, increase frictional heat, and reduce temperature drop at the center and edges of the strip. For example, the reduction rate for stand F6 can be 27%, 27.4%, 27.8%, 28.2%, 28.6%, 29%, etc.; and the reduction rate for stand F7 can be 17%, 17.4%, 17.8%, 18.2%, 18.6%, 19%, etc.

[0061] Ultra-rapid cooling of hot-rolled coils is an advanced metalworking technology designed to improve the microstructure, mechanical properties, and surface quality of hot-rolled materials. In this process, a high-pressure cooling medium (such as water or oil) is sprayed at high speed onto the surface of the hot-rolled coil through a specialized nozzle system, achieving rapid heat transfer in an extremely short time, thereby effectively controlling the grain size and residual stress distribution within the material. In the embodiments of this application, the hot-rolled coil is subjected to ultra-rapid cooling. After the strip completes its phase transformation, it is rapidly cooled, resulting in finer and more uniform grains at the edges, avoiding waviness defects caused by differences in edge microstructure after subsequent cold rolling.

[0062] In some embodiments, the pressure of the ultra-rapid cooling is 0.45 MPa to 0.55 MPa.

[0063] When the pressure of ultra-rapid cooling is in the range of 0.45 MPa to 0.55 MPa, it ensures that the cooling medium is sprayed onto the hot-rolled coil at an appropriate speed and flow rate, thereby achieving a uniform cooling effect. If the pressure is too low, it may lead to insufficient cooling, affecting product quality and performance; while if the pressure is too high, it may increase energy consumption and equipment wear. For example, the pressure of ultra-rapid cooling can be 0.45 MPa, 0.47 MPa, 0.49 MPa, 0.51 MPa, 0.53 MPa, 0.55 MPa, etc.

[0064] In some embodiments, the ultra-fast cooling rate is 70°C / s to 100°C / s.

[0065] If the cooling rate is too slow, the temperature gradient inside the hot-rolled coil may not be large enough, thus failing to achieve the desired cooling effect and grain refinement. Conversely, if the cooling rate is too fast, although the temperature can be reduced rapidly, it may also cause other problems, such as deformation or cracking due to excessive thermal stress. Therefore, setting the cooling rate within the range of 70℃ / s to 100℃ / s ensures that the hot-rolled coil can achieve rapid temperature reduction during the cooling process while avoiding the negative effects of excessively rapid cooling. This rate range meets the requirements for grain refinement while ensuring product quality and performance. For example, ultra-fast cooling rates can be 70℃ / s, 75℃ / s, 80℃ / s, 85℃ / s, 90℃ / s, 95℃ / s, 100℃ / s, etc.

[0066] S4. The hot-rolled coil is cold-rolled to obtain a cold-rolled sheet.

[0067] In summary, by optimizing in-furnace heating process parameters, rationally utilizing edge heating technology to maintain uniform strip edge temperature, optimizing the finishing mill roll cooling system to ensure stable roll operation, scientifically allocating load distribution among finishing mill stands, and activating ultra-fast cooling medium-pressure mode, the overall shape quality of IF steel cold-rolled sheets can be significantly improved. These strategies not only help enhance product market competitiveness and meet downstream users' demands for high-quality products, but also effectively reduce energy consumption and cost inputs during production, thereby increasing production efficiency. Therefore, in future production practices, we should continue to explore and improve the application strategies of these advanced technologies in daily production, and actively promote them to a wider range of steel product production lines to drive overall technological progress and industrial upgrading in my country's steel industry.

[0068] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0069] This application provides a method for improving the shape of IF steel cold-rolled sheet, comprising:

[0070] The billets are heated in a furnace, and the gap between adjacent billets is controlled; wherein the furnace heating includes: a preheating section, a first heating section, a second heating section, and a soaking section;

[0071] The heated billet is then subjected to edge heating to ensure that the target area of ​​the billet reaches the set temperature.

[0072] The heated billet is then subjected to precision rolling and ultra-rapid cooling to obtain a hot-rolled coil; wherein the work rolls of the precision rolling are subjected to stepped cooling.

[0073] The hot-rolled coil is then cold-rolled to obtain a cold-rolled sheet. Specific process parameters are shown in Table 1.

[0074] Table 1

[0075]

[0076] The cold-rolled sheet shape degradation rates for the examples and comparative examples are shown in Table 1.

[0077] The cold-rolled sheet shape degradation rate is an important indicator for measuring the quality of cold-rolled sheet shape. It refers to the proportion of products downgraded due to shape defects (such as waviness, warping, edge thinning, etc.) during the cold rolling production process. The calculation method is: Degradation rate = (Number of downgraded products / Total production quantity) × 100%.

[0078] As shown in Table 1, the preparation process parameters of the embodiments are all within the required range of the present invention, and the shape defects of the cold-rolled sheet are well controlled.

[0079] Appendix Figure 2-3 Detailed explanation:

[0080] Figure 2 Edge microstructure of hot-rolled coils provided as comparative examples in this application; Figure 3 The edge structure of the hot-rolled coil provided in the embodiments of this application. (By...) Figure 2-3 It is understood that the edge structure of the hot-rolled coil provided in this application embodiment is more uniform, which can avoid the waviness defects after cold rolling caused by edge structure differences.

[0081] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0082] The cold-rolled IF steel sheet provided in this embodiment of the invention has a greatly improved sheet shape, a smoother and flatter surface with no obvious unevenness, and uniformity throughout the width direction.

[0083] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for improving the shape of IF steel cold-rolled sheet, the method comprising: The billets are heated in a furnace, and the gap between adjacent billets is controlled; wherein the furnace heating includes: a preheating section, a first heating section, a second heating section, and a soaking section; The heated billet is then subjected to edge heating to ensure that the target area of ​​the billet reaches the set temperature. The heated billet is then subjected to precision rolling and ultra-rapid cooling to obtain a hot-rolled coil; wherein the work rolls of the precision rolling are subjected to stepped cooling. The hot-rolled coil is then cold-rolled to obtain a cold-rolled sheet; The temperature of the heat spreader is 1170℃~1190℃; The gap between adjacent castings is 200mm to 280mm; The temperature of the heat exchange section is more than 20°C higher than the temperature of the second heat exchange section; The target area is within 80mm of the edge; The set temperature is 50℃~80℃; The edge roller of the working roller is the area within 200mm of the edge of the working roller. Through the stepped cooling, the temperature of the edge roller is 15℃ to 20℃ lower than the temperature of the middle roller of the working roller.

2. The method according to claim 1, characterized in that, The reduction rate of the finishing mill stand F6 is 27% to 29%, and the reduction rate of the finishing mill stand F7 is 17% to 19%.

3. The method according to claim 1, characterized in that, The pressure of the ultra-rapid cooling is 0.45MPa to 0.55MPa.

4. The method according to claim 1, characterized in that, The ultra-fast cooling rate is 70℃ / s to 100℃ / s.

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