A Flexible Control Method for the Failure of Hot-Rolled Steel Strip Flat Coils

Abstract

The present application provides a flexible control method for the failure of flat coils of hot-rolled strip steel, which includes: Step 1, input the steel grade and evaluate the flat coil condition of the steel grade; Step 2, calculate or measure the Ar3 temperature in the phase transformation temperature of the steel grade; Step 3, obtain the isothermal transformation law of austenite, cool the hot-rolled strip steel by a segmented cooling method, and quickly cool the hot-rolled strip steel to the nose point T of ferrite, pearlite or bainite transformation by the front-stage rapid cooling method nose , and calculate the austenite phase transformation rate when the hot-rolled strip steel reaches the coiler mandrel #imgabs0#. Increase the austenite phase transformation rate by adjusting process parameters #imgabs1#, and perform a logical judgment on the austenite phase transformation rate #imgabs2#. When the austenite phase transformation rate is #imgabs3#, output the second flat coil control process; when the austenite phase transformation rate is #imgabs4#, output the third flat coil control process. This method can select different flat coil control processes and effectively control the occurrence of flat coils of the strip steel.

Description

Technical Field

[0001] The present application relates to the field of hot-rolled steel strip, and in particular to a flexible control method for failure of a hot-rolled steel strip flat coil. Background Art

[0002] Hot-rolled strip products are gradually developing towards high strength and thin specifications. Some high-strength steel varieties have flat coil failure problems during the production process due to their high hardenability alloy content and thin thickness, which hinders the stable mass production of products. Flat coil failure of hot-rolled steel coils (referred to as flat coils) refers to the phenomenon of instability in which the hot-rolled steel coils collapse under the action of gravity after coiling and unloading and become elliptical. The flat coils cannot be smoothly put on the uncoiler in the subsequent process due to the collapse of the inner ring, and additional manual processing is required, which increases labor intensity and production costs, and also affects production efficiency and product yield.

[0003] In response to the flat coil problem of hot-rolled strip steel, the patent with publication number CN1506174A discloses a control method, which controls the coiling temperature of steel strip with a carbon content of more than 0.25% within the range of Ar1+ (-10 to +60°C), so that the phase transformation of the steel coil gradually eliminates the flat coil from the outer layer and the core part where the coil contacts the reel to the middle layer. The patent with publication number CN108754104A discloses a method for the defects of dual-phase steel flat coils, in which the hot rolling final rolling temperature is controlled according to the starting temperature of the phase transformation of austenite to ferrite, promoting the transformation of austenite to ferrite; the hot rolling cooling section is divided into a fast cooling section and a slow cooling section, in the fast cooling section, the hot-rolled plate is cooled to the intermediate temperature of the layer cooling, promoting the transformation of austenite to ferrite, and in the slow cooling section, promoting the transformation of austenite to pearlite, reducing the proportion of residual austenite. The patent with publication number CN107904376A provides a method and device for preventing flat coils of dual-phase steel. By controlling the finishing rolling outlet temperature to 860-900°C and the cooling rate to 70-100°C / s, energy is accumulated for the phase change of the strip steel so that the phase change of the strip steel occurs in advance, preventing the strip steel from undergoing phase change and forming flat coils after being coiled. The patent with publication number CN102335681A discloses a coiling method for preventing flat coils of hot-rolled strip steel. The coiling temperature is 500-600°C, and then the coiled steel coil is left on the coiler drum for 20s-60s. The patent with publication number CN107812789A discloses a method for preventing flat coils, in which the hot-rolled strip is laminar-cooled and rapidly cooled to 650-700°C or 500-550°C for coiling, and the hot-rolled steel coil after coiling stays on the coiler for 5-20 seconds; the patent with patent number ZL202010183210.4 discloses a method for controlling flat coils of hot-rolled advanced high-strength steel, which controls the flat coils through the strip finishing surface roughness control stage, the laminar cooling austenite phase transformation control stage, the coiling tension control stage, and the rotation control stage after coiling.

[0004] The above-mentioned patents all believe that the phase transformation expansion caused by the phase transformation after the coiling of the steel coil is the reason for the generation of flat coils. However, this theory cannot explain the phenomenon that the original flat coil steel grades do not produce flat coils after adopting the coiling process in the austenite region during production practice. Because the austenite region coiled steel coil will definitely undergo the temperature drop phase transformation of austenite during the cooling process, resulting in expansion. In fact, the strip steel will only produce flat coils when it is in the phase transformation stage within the limited length between the coiling pinch roll and the mandrel (that is, the situation where the strip steel undergoes phase transformation while being coiled); therefore, the temperature of the strip steel when it reaches the mandrel is a more accurate flat coil control parameter than the CT temperature. So, the existing technology is not designed in combination with the actual characteristics of the laminar roller table length of the production line, nor does it consider the temperature drop process from the coiling thermometer to the steel coil, and it cannot solve the flat coil problems caused by different specifications of the same steel grade. Summary of the Invention

[0005] One of the purposes of this application is to provide a flexible control method for the failure of flat coils of hot-rolled strip steel to solve the problem that the existing hot-rolled strip steel is prone to generating flat coils.

[0006] The technical solution of this application is as follows:

[0007] A flexible control method for the failure of flat coils of hot-rolled strip steel includes the following steps:

[0008] Step 1: Input the steel grade and evaluate the flat coil condition of the steel grade by distinguishing the thickness specifications. If the steel grade is a new steel grade, perform the operations in Step 2; if the steel grade has been produced, measure the inner diameter of the steel coil and make a logical judgment on the inner diameter size of the steel coil. When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil is less than 1.05, the coil shape of the steel coil is good, and the original production process is output (the original production process means that the steel grade has been produced, and the steel grade will not generate flat coils without process adjustment using the original production process); when the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil is greater than or equal to 1.05, there is a flat coil problem that needs to be controlled for the steel coil, and perform the operations in Step 2;

[0009] Step 2: Calculate or measure the start temperature Ar3 temperature of the transformation from austenite to ferrite during the cooling of the steel in the phase transformation temperature of the steel grade. When the coiling temperature of the steel grade can be set to a temperature higher than Ar3 + 20°C, calculate the austenite phase transformation rate of the hot-rolled strip steel when it reaches the coiling mandrel with Ar3 + 20°C as the coiling temperature target value By adjusting multiple parameters of the hot-rolled strip steel during the hot-rolling process to make the austenite phase transformation rate decrease, and make a logical judgment on the austenite phase transformation rate When the austenite phase transformation rate is..., output the first flat coil control process; when the austenite phase transformation rate is..., perform the operations in Step 3;

[0010] Step 3: Obtain the isothermal transformation law of austenite through software simulation or thermal simulation tests, cool the hot-rolled strip by means of segmented cooling, and rapidly cool the hot-rolled strip to the nose point T of ferrite, pearlite or bainite transformation in the isothermal transformation curve of the hot-rolled strip nose , and calculate the austenite phase transformation rate when the hot-rolled strip reaches the coiler mandrel Increase the austenite phase transformation rate by adjusting process parameters And perform a logical judgment on the austenite phase transformation rate . When the austenite phase transformation rate is reached, output the second flat coil control process; when the austenite phase transformation rate is reached, then output the third flat coil control process

[0011] As a technical solution of the present application, in Step 2, the starting temperature Ar3 temperature of the austenite to ferrite transformation of the steel grade is calculated by an empirical formula or a phase transformation simulation software, where the empirical formula is:

[0012] Ar3 = 901 - 325×C% + 33×Si% + 290×P% + 45×Al% - 92×Mn% - 48×Cr%

[0013] As a technical solution of the present application, in Step 2, the Ar3 of the steel grade is measured through a thermal simulation test

[0014] As a technical solution of the present application, in Step 2, the austenite phase transformation rate is calculated by combining the finishing temperature and finishing speed of the steel grade, the characteristics of the laminar flow and coiling line length, and the product thickness specification

[0015] As a technical solution of the present application, in Step 3, the austenite phase transformation rate is calculated by combining the finishing temperature and finishing speed of the steel grade, the characteristics of the laminar flow and coiling line length, and the product thickness specification

[0016] As a technical solution of the present application, in Step 2, the austenite phase transformation rate is reduced by adjusting the parameters of the finishing temperature of the hot-rolled strip, the rolling speed of the hot-rolled strip, and the distance from the finishing mill exit to the coiler of the hot-rolled strip And the parameters of the finishing temperature of the hot-rolled strip, the rolling speed of the hot-rolled strip, and the distance from the finishing mill exit to the coiler of the hot-rolled strip output in the first flat coil control process all satisfy that the temperature T of the hot-rolled strip to the mandrel 卷筒 ≥Ar3, or T 卷筒 <Ar3 and the austenite phase transformation rate

[0017] As a technical solution of the present application, in step three, the second flat coil control process is to adopt a segmented cooling method for the hot-rolled strip steel and quickly cool the hot-rolled strip steel to the nose point T of ferrite, pearlite or bainite transformation in the isothermal transformation curve of the strip steel through rapid cooling nose , so that the phase transformation rate of the strip steel austenite on the laminar roller table reaches the highest, and the rolling speed, intermediate cooling temperature, and the selected coiler output in the second flat coil control process all satisfy the austenite phase transformation rate of the hot-rolled strip steel when it reaches the reel

[0018] As a technical solution of the present application, in step three, the third flat coil control process is to control the coiling temperature CT at the head length L of the hot-rolled strip steel, and quickly cool the hot-rolled strip steel to the temperature range where austenite undergoes martensite transformation at a temperature of ≥150°C / s, and make the coiling temperature CT ≤ T 90m , where T 90m is the temperature when 90% martensite transformation occurs in austenite; for different strengths of the hot-rolled strip steel, coil weight, and thickness of the hot-rolled strip steel, then L = G / 22×75000 / (σh)×2.0 / h×1250 / W, where G is the coil weight, σ is the yield strength of the hot-rolled strip steel at T 90m , h is the thickness of the hot-rolled strip steel, and W is the width of the hot-rolled strip steel.

[0019] As a technical solution of the present application, in step two, the weight percentage content of the chemical components of the hot-rolled strip steel is: C: 0.005 - 0.80%; Si: 0.01 - 2.0%; Mn: 0.01 - 3.0%; Cr ≤ 1.0%; Cu ≤ 0.5%; Ti + Nb + V + Mo ≤ 1.0%; P ≤ 0.03%; S ≤ 0.01%; N ≤ 0.01%; B ≤ 0.01%; the rest is Fe and inevitable impurities.

[0020] In addition, the control flow chart of a flexible control method for the failure of flat coils of hot-rolled strip steel provided by the present application is as Figure 1 shown. The reasons for adopting the above technical solutions will be elaborated below in combination with the flat coil mechanism of the hot-rolled strip steel. It should be noted that there are three positions and stages where the phase transformation of the strip steel is completed during the hot-rolling production process, as Figure 2 shown:

[0021] (1) The strip steel is on the laminar roller table: it is stage ①;

[0022] (2) The strip steel is between the pinch roll and the coiler reel, and for a period of time after entering the coiler (such as within the range of 1 - 3 coils of strip steel), it is stage ②;

[0023] (3) The strip is in the transport chain behind the lower coiler and stored in the coil warehouse, which is stage ③;

[0024] When the strip is in the phase change stage at stage ②, flat coils are likely to be produced (i.e., phase change occurs while coiling). Flat coils will not occur when the phase change is completed at stage ① or when the phase change begins at stage ③.

[0025] The relevant mechanism of the flat coil produced by the phase change of the strip in stage ② is as follows:

[0026] (1) When the strip is coiled, the phase change expansion toward the tail end of the coil will interfere with the tension of the pinch roller, resulting in coiling tension loss, such as Figure 3 (2) The strip is in phase change stage between the pinch roller and the drum, and for a period of time after entering the coiler (e.g., 1-3 turns of strip), and the strip undergoes phase change expansion and phase change plastic elongation. This elongation process occurs simultaneously with the coiling process, making it difficult to coil the steel coil tightly. As a result, the radial compressive stress between layers is reduced, and the friction between layers is reduced, so that the strip is prone to interlayer slippage and reduced stiffness. Figure 4 After unloading, the steel coil becomes flat due to its own weight.

[0027] Most alloying elements in steel shift the phase transformation C curve to the right, increase the stability (Hardenability) of supercooled austenite, and prolong the incubation period of ferrite, pearlite and bainite phase transformation; the more alloying elements in steel, the more significant the effect. According to the mechanism of hot-rolled flat coil, by controlling the cooling path of the strip, that is, by intervening in phase transformation, the phase transformation of the strip is avoided as much as possible in stage ②, which is the direction of flat coil control, that is, the phase transformation is completed in the laminar roller (stage ①) or the phase transformation starts completely in stage ③.

[0028] Therefore, in step 1, when the ratio of the inner diameter major axis a to the minor axis b of the steel coil is ≤1.05, the steel coil has a good shape, indicating that the strip steel has not undergone phase change in stage ②, or after a small amount of phase change, the strip steel is thicker and the inner ring has greater rigidity, which supports the weight of the steel coil. At this time, the shape of the steel coil produced by the strip production process is acceptable and does not need to be adjusted. When the ratio of the inner diameter major axis a to the minor axis b of the steel coil is greater than 1.05, it means that the steel coil has a flat coil problem that needs to be controlled.

[0029] In step 2, a flat coil control process based on austenite single phase coiling is proposed. First, the phase change temperature Ar3 of the steel grade is calculated or measured, and then the CT temperature of the steel grade is logically judged. When the CT temperature of the steel grade can be set to a temperature higher than Ar3+20℃, it means that this steel grade can be controlled by the austenite coiling process and the strip begins to change phase in stage ③. The reason for setting Ar3+20℃ as the logical judgment parameter is that the strip reaching the coil temperature Tcoil is a more reasonable and critical flat coil control parameter than CT. However, the CT thermometer of the hot rolling production line is still some distance away from the coiler, that is, the strip still has a cooling process between the CT thermometer and the coiler. Therefore, setting CT to ≥Ar3+20℃ makes Tcoil ≥Ar3 to prevent flat coiling. However, due to the long laminar flow section of some hot rolling production lines, when the strip thickness is thin (the air cooling speed is high), even if the final rolling temperature is increased, the strip head plate speed and rolling speed are increased, and the laminar flow roller is air-cooled over the entire length, it is still impossible to meet T reel ≥ Ar3, or meet T reel slightly lower than Ar3 but the austenite phase transformation rate is At this time, it is impossible to control the flat coil by the control process based on austenite single phase coiling. In addition, for some steel grades, based on the requirements of microstructure performance or surface oxidation degree, CT cannot be set to a temperature higher than Ar3+20℃, and at this time, it is also impossible to control the flat coil by the control process based on austenite single phase coiling.

[0030] When the control process based on austenite single phase coiling described in step 2 cannot be used or the relevant conditions cannot be met by using this process, the medium temperature coiling control process based on MT (intermediate temperature) control described in step 3 can be used. There is a temperature range with a faster transformation speed in the isothermal transformation of austenite in steel, that is, the nose point T of the phase transformation C curve. nose The relevant curve law (T nose , incubation period, 50%, 100% transformation time), and then determine through logical judgment whether the MT of the steel grade can be set to MT=T nose If the structural properties of the steel grade allow MT = T nose , the cooling path of the strip can be controlled by this process, and the strip can be quickly cooled to the ferrite, pearlite or bainite transformation nose point T by the front stage rapid cooling. nose , and then air-cooled for a period of time to allow the strip to complete most of the austenite transformation on the laminar roller. Different steel grades, final rolling temperature and speed, laminar flow and coiling line length characteristics, and product specifications will affect the austenite transformation rate of the strip reaching the reel. Lower alloy composition, lower final rolling speed, longer laminar roller, and thicker strip thickness (affecting rolling speed) are all beneficial to the austenite transformation rate. Increase. Increase by adjusting process parameters when satisfy When the medium temperature coiling flat coil control process based on MT control cannot be used, or the austenite phase transformation rate on the laminar roller cannot be met by using this process, When , the third flat roll control process is output.

[0031] The third flat coil control process means that it is impossible to avoid phase change of the strip as much as possible by phase change intervention. At this time, it is necessary to increase the stiffness of the inner ring of the steel coil to prevent the flat coil from occurring under the weight of the steel coil after unloading. The idea of ​​the third flat coil control process in step 3 is to control the coiling temperature CT of a certain length L of the strip head, and cool the strip head at a speed of ≥150℃ / s to make CT≤T 90m , T 90m Calculated by Jmatpro software or measured by thermal simulation test. L is determined according to the strip strength, coil weight and thickness and width specifications, L = G / 22×75000 / (σh)×2.0 / h×1250 / W, where G is coil weight (tons) and σ is T 90m The strip yield strength (MPa) is also calculated by Jmatpro software, where h is the strip thickness (mm) and W is the strip width (mm). The strength of the strip is improved by rapidly cooling a certain length of the strip at the head into the martensite zone, and the coiling forms a high-strength core shaft to support the weight of the steel coil.

[0032] Beneficial effects of this application:

[0033] In the flexible control method of hot-rolled flat coil failure of the present application, it is a flexible control method of flat coil failure based on the laminar roller length characteristics of the hot rolling production line, the steel grade composition and thickness specifications, and the strip cooling conditions; the method can flexibly select the flat coil control process based on austenite single-phase zone coiling or the flat coil control process based on intermediate temperature control according to the phase change law of the steel grade, the temperature and speed of the strip, the length of the laminar roller of the production line and other parameters, and the austenite phase change rate is regulated so that the strip is not in the phase change process for a period of time after the pinch roller to the coiler reel, thereby controlling the flat coil. In addition, it also provides a steel coil inner ring stiffness method, which forms a high-strength mandrel to support the weight of the steel coil by increasing the inner ring stiffness of the steel coil of a certain thickness. In addition, the flat coil flexible control method provided by the method is suitable for different hot rolling production lines, comprehensively covers different steel grades and thickness specifications, has strong adaptability, and can reduce the yield rate and quality degradation loss caused by flat coils. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] To more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0035] Figure 1 Flowchart of the flexible control method for the failure of flat coils of hot-rolled strip steel provided by the first embodiment of the present application;

[0036] Figure 2 Schematic diagram of different positions and different stages of phase transformation completion during hot-rolling production of strip steel provided by the first embodiment of the present application;

[0037] Figure 3 Schematic diagram of the phase transformation expansion interfering with the pinch roll tension in the flat coil mechanism provided by the first embodiment of the present application;

[0038] Figure 4 Schematic diagram of the coiling losing tension caused by phase transformation expansion and phase transformation plastic elongation in the flat coil mechanism provided by the first embodiment of the present application;

[0039] Figure 5 Schematic diagram of the length characteristics of the laminar roller table of the production line provided by the first embodiment of the present application;

[0040] Figure 6 Schematic diagram of the CCT curve of 22MnB5 provided by the first embodiment of the present application;

[0041] Figure 7 Schematic diagram of the TTT curve of HR980QP provided by the third embodiment of the present application.

[0042] Icon: 1 - Strip steel; 2 - Coiler. Specific Embodiments

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Usually, the components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0044] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the present application to be protected, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present application.

[0045] It should be noted that similar reference numerals and letters indicate similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0046] In the description of the present application, it should be noted that the orientation or positional relationship indicated by terms such as "upper", "lower", etc. is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the inventive product is usually placed during use. It is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application.

[0047] First Embodiment:

[0048] Steel grade for production: 22MnB5. The chemical composition (wt.%) of strip 1 is shown in Table 1, and the rest is Fe and inevitable impurities; the length characteristics of the laminar roller table in the production line are as Figure 5 and shown in Table 2. The thickness specification of strip 1 ranges from 1.0 mm to 4.0 mm.

[0049] Table 1: Chemical Composition of 22MnB5 (wt.%)

[0050] C Si Mn P S N Cr B Alt 0.22 0.25 1.40 0.008 0.002 0.004 0.20 0.004 0.04

[0051] Table 2: Length Characteristics of the Laminar Roller Table in the Hot Rolling Production Line

[0052] Production line type L ΔL1 ΔL2 ΔL3 CSP production line 90m 12m 21m 30m

[0053] Step 1: Evaluate the flat coiling condition of the 22MnB5 steel grade according to the thickness specification, as shown in Table 3. For this steel grade, the ≥1.2 mm specification has been produced in the production line. Measure the inner diameter of the steel coil and make a logical judgment on the inner diameter size. When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil ≤ 1.05, it indicates that the coiling shape of the steel coil is good, and the original production process is directly output; when the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil > 1.05, it means that there are flat coiling problems that need to be controlled for the steel coil. The 1.0 mm specification steel coil has not been produced, but the thinner the specification, the greater the risk of flat coiling. For 22MnB5 steel, flat coiling occurs when the ≤2.0 mm specification adopts the original laminar cooling process, and optimization control is required; the 22MnB5 steel grade with a ≤2.0 mm specification enters the operation in Step 2;

[0054] Table 3: Evaluation of the Flat Coiling Condition of 22MnB5

[0055]

[0056] Step 2: First, measure the transformation temperature Ar3 of 22MnB5. Through thermal simulation experiments, Ar3 = 710 °C is obtained. As Figure 6 shown in the CCT curve; then, perform a logical judgment on the CT temperature setting. Since the final properties of 22MnB5 steel are determined by the subsequent hot forming process and it has a relatively high tolerance for surface internal oxidation, its CT temperature can be set to a temperature higher than 710 + 20 °C.

[0057] Perform laminar cooling control with 730 °C as the CT target value. Calculate the austenite transformation rate of strip 1 when it reaches the coiler 2 mandrel according to the temperature-time curve using phase transformation simulation software (such as JMatPro). By adjusting the finishing rolling temperature and rolling speed, select parameters such as coiler 2 (i.e., different distances from the finishing mill exit to coiler 2) to make the temperature of strip 1 reaching the mandrel ≥ 710 °C as much as possible, so as to reduce the austenite transformation rate. After that, perform a logical judgment on the austenite transformation rate. As shown in Table 4, it can be seen that when different coiler 2 are selected under the same thickness condition of strip 1, the austenite transformation rate of strip 1 when it reaches the mandrel is different; when it meets Output the first flat coil control process. As shown in Table 5, it can be seen that the flat coil of 22MnB5 strip 1 can be controlled through the control process based on coiling in the austenite single-phase region. Output the first flat coil control process. As shown in Table 5, it can be seen that the flat coil of 22MnB5 strip 1 can be controlled through the control process based on coiling in the austenite single-phase region.

[0058] Table 4: Phase transformation simulation calculation results and coil shape control performance of 22MnB5 process optimization

[0059]

[0060] Table 5: Output flat coil control process of 22MnB5

[0061]

[0062] Second Embodiment:

[0063] Production steel grade: 65Mn. The chemical composition (wt.%) of strip 1 is shown in Table 6, and the rest is Fe and inevitable impurities. The characteristics of the production line are as Figure 5 shown in Table 2, and the thickness specification of strip 1 ranges from 1.4 mm to 4.0 mm.

[0064] Table 6: Chemical composition of strip 1 (wt.%)

[0065] C Si Mn P S N Alt 0.66 0.20 1.00 ≤0.012 ≤0.005 0.005 0.03

[0066] Step 1: Evaluate the flat coil condition of 65Mn steel by thickness specifications as shown in Table 7. For the ≥2.0 mm specification of this steel grade, production has already been carried out on the production line. Measure the inner diameter of the steel coil and make a logical judgment on the inner diameter size. When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil ≤ 1.05, it indicates that the coil shape is good, and the original production process is directly output. When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil > 1.05, it means that there are flat coil problems that need to be controlled. For 65Mn steel, flat coils occur when using the original laminar cooling process for the ≤2.0 mm specification, and control is required. The ≤2.0 mm specification then enters Step 2 for operation;

[0067] Step 2: First, measure the phase transformation temperature Ar3 of 65Mn. Through phase transformation simulation software, Ar3 = 715 °C is obtained. Then, make a logical judgment on the CT temperature setting. Since the 65Mn steel is subsequently cold-rolled in thin specifications by users, there are large fluctuations in the properties of the head, middle, and tail of the hot-rolled coil, and there is a risk of strip breakage. Therefore, in this case, the CT temperature cannot be set to CT = Ar3 + 20 °C. When the flat coil process based on coiling in the single-phase austenite region described in Step 2 cannot be adopted, the medium-temperature coiling control process based on MT control in Step 3 can be used.

[0068] Table 7: Evaluation of 65Mn Flat Coil Condition

[0069]

[0070] Step 3: Obtain the isothermal transformation law of 65Mn austenite through thermal simulation experiments, and obtain the pearlite transformation nose tip temperature T nose = 570 °C. Since the strength is higher at lower phase transformation temperatures, strip 1 is cooled in a segmented manner, quickly cooled to MT = 570 - 610 °C above the nose tip at a large cooling rate strategy, and then air-cooled without controlling CT. Combine the finish rolling temperature and speed of the steel grade, the characteristics of the laminar flow and coiling line lengths, and the product specifications to calculate the phase transformation rate of 65Mn strip 1 when it reaches the mandrel of coiler 2 as shown in Table 8; increase through process parameter adjustment Then perform a logical judgment on the austenite phase transformation rate . When the phase transformation rate of the laminar roller table is large, the release of the latent heat of phase transformation causes the temperature of strip 1 to rise, and CT naturally evolves to be higher than MT. When meets , output the second flat coil control process as shown in Table 9; it can be seen that the flat coils of 65Mn strip 1 can be controlled through the medium-temperature coiling control process based on MT control.

[0071] Table 8: Phase Transformation Simulation Calculation Results and Coil Shape Control Performance of 65Mn Process Optimization

[0072]

[0073]

[0074] Table 9: 65Mn Output Flat Coil Control Process

[0075]

[0076] The Third Embodiment:

[0077] Steel grade for production: HR980QP; The chemical composition (wt.%) of strip 1 is shown in Table 10, and the rest is Fe and unavoidable impurities; The production line characteristics are as Figure 5 and Table 2 show that the thickness specification of strip 1 ranges from 1.4 mm to 4.0 mm, and the width ranges from 1000 to 1600 mm.

[0078] Table 10: Chemical Composition (wt.%) of HR980QP Strip 1

[0079] C Si Mn P S N Alt 0.19 1.82 1.91 0.008 0.002 0.004 0.03

[0080] Step 1: Evaluate the flat coil condition of HR980QP steel grade by thickness specification, as shown in Table 11. For the steel grade with a specification of ≥1.4 mm, production has been carried out on the production line. Measure the inner diameter of the steel coil and make a logical judgment on the inner diameter size; When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil ≤ 1.05, it indicates that the coil shape is good, and the original production process is directly output; When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil > 1.05, it means that there is a flat coil problem that needs to be controlled; The HR980QP steel grade with a specification of ≤ 2.0 mm enters the operation in Step 2;

[0081] Step 2: First, calculate the Ar3 temperature of the phase transformation temperature of HR980QP by an empirical formula, and it is calculated that Ar3 = 727 °C; Then make a logical judgment on the CT temperature setting. Since the subsequent process of HR980QP steel is cold rolling and galvanizing, its surface cannot accept internal oxidation, so the CT temperature cannot be set to a temperature higher than 727 + 20 °C.

[0082] Table 11: Evaluation of HR980QP Flat Coil Condition

[0083]

[0084] Step 3: Obtain the isothermal transformation law of austenite of HR980QP through phase transformation software simulation, as Figure 7 , obtain the pearlite nose point temperature T nose= 580 °C. The strip steel 1 is cooled in a segmented manner and quickly cooled to MT = 580 - 600 °C by a large cooling rate strategy. Based on the requirements of internal oxidation and performance control, CT is limited to 500 °C. Therefore, after cooling to MT, it is air-cooled for a period of time and then water-cooled to 500 °C for coiling. Combining the finish rolling temperature and speed of the steel grade, the characteristics of the laminar flow and coiling line length, and the product specifications, calculate the austenite phase transformation rate of the strip steel 1 of HR980QP when it reaches the mandrel of the coiler 2. As shown in Table 12; therefore, due to the phase transformation law and CT temperature limitation of the HR980QP steel grade, the medium-temperature coiling process based on MT control cannot control flat coiling.

[0085] Table 12: Process optimization of HR980QP, phase transformation simulation calculation results, and actual results of coiling shape control

[0086]

[0087] Step 4: Adopting Step 4 means that it is impossible to make the phase transformation of the strip steel 1 of HR980QP avoid stage ② as much as possible through phase transformation intervention. At this time, it is necessary to adopt a method of increasing the stiffness of the inner coil of the steel coil to prevent flat coiling under the action of the self-weight of the steel coil after uncoiling. First, calculate through the phase transformation simulation calculation software to obtain T 90m = 245 °C. At this temperature, the yield strength σ of the steel is 644 MPa. Then, quickly cool a section with a length of L at the head of the strip steel 1 to CT so that CT ≤ T 90m , L = G / 22 × 75000 / (σh) × 2.0 / h × 1250 / W, where G is the coil weight (tons), σ is the yield strength of the strip steel 1 at T 90m (MPa), h is the thickness of the strip steel 1 (mm), and W is the width of the strip steel 1 (mm); it can be seen that the flat coiling of the strip steel 1 of HR980QP can be controlled by the inner coil stiffness method.

[0088] Table 13: Coiling control process of HR980QP and actual results of coiling shape control

[0089]

[0090] The above is only the preferred embodiment of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A flexible control method for the failure of flat coils of hot-rolled strip steel, characterized in that, it includes the following steps: Step 1, input the steel grade, and evaluate the flat coil condition of the steel grade by distinguishing the thickness specification: if the steel grade is a new steel grade, perform the operations in Step 2; if the steel grade has been produced, measure the inner diameter of the steel coil and perform a logical judgment on the inner diameter size of the steel coil. When the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil is less than 1.05, the coil shape of the steel coil is good, and the original production process is output; when the ratio of the major axis a to the minor axis b of the inner diameter of the steel coil is greater than or equal to 1.05, there is a flat coil problem that needs to be controlled for the steel coil, and perform the operations in Step 2; Step 2: Calculate or measure the Ar3 temperature, which is the starting temperature of the austenite-to-ferrite transformation during the cooling of the steel, in the phase transformation temperature of the steel grade: When the coiling temperature of the steel grade can be set to a temperature higher than Ar3 + 20°C, then use Ar3 + 20°C as the coiling temperature target value to calculate the austenite phase transformation rate when the hot-rolled strip reaches the coiler mandrel. Reduce the austenite phase transformation rate by adjusting multiple parameters during the hot rolling process of the hot-rolled strip. And perform a logical judgment on the austenite phase transformation rate. When the austenite phase transformation rate is reached, then output the first flat coiling control process; when the austenite phase transformation rate is reached, then perform the operations in Step 3. Step 3: Obtain the isothermal transformation law of austenite through software simulation or thermal simulation tests. Cool the hot-rolled strip by means of segmented cooling, and rapidly cool the hot-rolled strip to the nose point T of ferrite, pearlite or bainite transformation in the isothermal transformation curve of the hot-rolled strip nose , and calculate the austenite phase transformation rate when the hot-rolled strip reaches the coiler mandrel Increase the austenite phase transformation rate by adjusting process parameters And conduct a logical judgment on the austenite phase transformation rate . When the austenite phase transformation rate , output the second flat coil control process; when the austenite phase transformation rate , output the third flat coil control process 2. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, in Step 2, the starting temperature Ar3 temperature of the austenite to ferrite transformation of the steel grade is calculated by an empirical formula or a phase transformation simulation software, where the empirical formula is Ar3 = 901 - 325×C% + 33×Si% + 290×P% + 45×Al% - 92×Mn% - 48×Cr%.

3. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, in Step 2, the Ar3 of the steel grade is measured by a thermal simulation test.

4. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, In step two, the austenite transformation rate is calculated by combining the finish rolling temperature and finish rolling speed of the steel grade, the characteristics of the laminar flow and coiling line length, and the product thickness specification.

5. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, In step three, the austenite phase transformation rate is calculated by combining the finish rolling temperature and finish rolling speed of the steel grade, the characteristics of the laminar flow and coiling line length, and the product thickness specification.

6. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 5, characterized in that, In Step 2, the austenite transformation rate is reduced by adjusting the parameters of the finishing temperature of the hot-rolled strip, the rolling speed of the hot-rolled strip, and the distance from the finishing mill outlet to the coiler of the hot-rolled strip Moreover, the parameters of the finishing temperature of the hot-rolled strip, the rolling speed of the hot-rolled strip, and the distance from the finishing mill outlet to the coiler of the hot-rolled strip output in the first flat coil control process all satisfy the temperature T of the hot-rolled strip reaching the reel 卷筒 ≥ Ar3, or T 卷筒 < Ar3 and the austenite transformation rate 7. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, In Step 3, the second flat coil control process is to adopt a segmented cooling method for the hot-rolled strip steel and rapidly cool the hot-rolled strip steel to the nose point T of ferrite, pearlite or bainite transformation in the isothermal transformation curve of the strip steel nose , so as to maximize the phase transformation rate of the strip steel austenite on the laminar roller table, and the rolling speed, intermediate cooling temperature, and the selected coiler output in the second flat coil control process all satisfy the austenite phase transformation rate of the hot-rolled strip steel when it reaches the reel 8. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, In Step 3, the third flat coiling control process is to control the coiling temperature CT at the position where the length of the head of the hot-rolled strip is L, and rapidly cool the hot-rolled strip to the temperature range where austenite undergoes martensitic transformation at a temperature of ≥150 °C / s, and make the coiling temperature CT ≤ T 90m , where T 90m is the temperature when 90% martensitic transformation occurs in austenite; for different strengths of the hot-rolled strip, coil weight, and thickness of the hot-rolled strip, then L = G / 22 × 75000 / (σh) × 2.0 / h × 1250 / W, where G is the coil weight, σ is the yield strength of the hot-rolled strip at T 90m , h is the thickness of the hot-rolled strip, and W is the width of the hot-rolled strip.

9. The flexible control method for the failure of flat coils of hot-rolled strip steel according to claim 1, characterized in that, in Step 2, the weight percentage content of the chemical components of the hot-rolled strip steel is: C: 0.005 - 0.80%; Si: 0.01 - 2.0%; Mn: 0.01 - 3.0%; Cr ≤ 1.0%; Cu ≤ 0.5%; Ti + Nb + V + Mo ≤ 1.0%; P ≤ 0.03%; S ≤ 0.01%; N ≤ 0.01%; B ≤ 0.01%; the rest is Fe and unavoidable impurities.

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