Method for preventing hot-rolled strip steel from being flattened and rolled

Abstract

The invention provides a method for preventing hot-rolled strip steel from being flattened, and belongs to the field of steel preparation. The method comprises the following steps: carrying out finish rolling on a rough rolled plate to obtain hot rolled strip steel; the hot-rolled strip steel is conveyed to a cooling line with a front section centralized cooling area, an air cooling area and a fine adjustment section water cooling area; the hot-rolled strip steel is subjected to front-section centralized cooling, so that the grain size of the strip steel is controlled; carrying out air cooling on the hot-rolled strip steel subjected to the front-section centralized cooling so as to carry out phase change at a relatively low cooling speed; in the fine adjustment section water cooling area, fine adjustment section water cooling is conducted on the air-cooled hot rolled strip steel, so that the structure performance of the hot rolled strip steel is improved; and the hot-rolled strip steel subjected to water cooling in the fine adjustment section is coiled, and a hot-rolled coil is obtained. The laminar cooling adopts a three-section cooling process of a front section centralized cooling area, an air cooling area and a fine adjustment section water cooling area, so that phase change of the strip steel is realized in advance before coiling under the condition of ensuring that the structure property meets the requirements, interlayer dislocation after coiling is reduced, and the flat coiling problem is obviously improved.

Description

Technical Field

[0001] The present application relates to the technical field of steel material preparation, and in particular to a method for preventing hot-rolled steel strip from flattening. Background Art

[0002] Carbon tool steel with a carbon content of more than 0.45% has the characteristics of high strength, high hardness, and excellent wear resistance. It is widely used in hardware knives, chains, automotive gaskets, sewing machine needles and other fields. Typical steel grades include 65Mn, C75S, SK85, etc. In order to obtain a good spheroidization rate and surface quality, there are special temperature requirements when producing this type of steel. The strip is prone to flattening after coiling, which seriously affects production efficiency and yield rate, and has become a problem that plagues the industry.

[0003] Studies have shown that the flat coil defect of medium and high carbon steel is mainly due to the phase change of the strip during or after coiling. The volume change of the phase change organization causes the interlayer displacement of the steel coil, resulting in coil collapse under the action of gravity. In order to improve the flat coil problem, technicians have made continuous efforts. The prior art proposes to control the cooling rate to improve the problem of hot-rolled flat coils. However, the cooling manifold capacity is effective, the range of cooling rate adjustment is limited, and the phase change point cannot be effectively controlled. At the same time, it is proposed to improve the problem of hot-rolled flat coils by controlling the convexity and wedge shape of the strip, and then matching the appropriate cooling method and coiling parameters. However, the adjustment of the coiling parameters increases the risk of strip narrowing. Therefore, how to develop a control method for hot-rolled flat coils has become a technical problem that needs to be solved urgently. Summary of the invention

[0004] The present application provides a method for preventing hot-rolled steel strip from flattening, so as to solve the following technical problem: how to reduce the occurrence rate of the problem of hot-rolled steel strip flattening.

[0005] The present application provides a method for preventing hot rolled steel strip from flattening, the method comprising:

[0006] At a set final rolling exit temperature, the rough rolled sheet is subjected to finish rolling to obtain a hot rolled steel strip;

[0007] The hot-rolled steel strip is conveyed to a cooling line having a front-stage concentrated cooling zone, an air cooling zone and a fine-adjustment water cooling zone;

[0008] In the front concentrated cooling zone, the hot rolled steel strip is subjected to front concentrated cooling, and the midpoint temperature of the cooling line is controlled to be a set midpoint temperature to control the grain size of the steel strip; the set midpoint temperature is greater than the bainite starting phase transformation temperature;

[0009] In the air cooling zone, the hot rolled steel strip is air cooled after the concentrated cooling in the previous stage to perform phase transformation at a lower cooling rate;

[0010] In the fine-adjustment section water cooling zone, the hot-rolled steel strip after air cooling is subjected to fine-adjustment section water cooling, and the final cooling temperature of the fine-adjustment section water cooling is controlled to be a set final cooling temperature, so as to improve the microstructure and properties of the hot-rolled steel strip; and

[0011] The hot-rolled steel strip after water cooling in the fine-tuning section is coiled to obtain a hot-rolled coil.

[0012] Optionally, the final rolling temperature is set to 820°C to 920°C.

[0013] Optionally, the midpoint temperature of the cooling line is the temperature of the hot-rolled strip at the middle temperature measurement point in the length direction of the cooling line, and the set midpoint temperature satisfies the following relationship:

[0014] MT1=BS+(10~50)℃

[0015] Wherein, MT1 represents the set midpoint temperature, and BS represents the bainite starting phase transformation temperature.

[0016] Optionally, the final cooling temperature is set to 500°C to 650°C.

[0017] Optionally, the cooling manifold water volume of the fine-tuning section water cooling is determined according to the final cooling temperature of the air cooling, the set final cooling temperature of the fine-tuning section water cooling and the latent heat of phase change.

[0018] Optionally, the final cooling temperature of the air cooling is 550°C to 650°C.

[0019] Optionally, the inlet temperature of the finish rolling is 1030°C to 1130°C.

[0020] Optionally, the method for preparing the rough rolled plate comprises:

[0021] The slab with a set chemical composition is heated and roughly rolled to obtain the rough-rolled plate.

[0022] Optionally, the rough rolling adopts a 3+3 mode.

[0023] Optionally, the heating time is 140 min to 300 min, and the heating steel tapping temperature is 1160°C to 1260°C.

[0024] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0025] The embodiment of the present application provides a method for preventing hot-rolled steel strip from flattening. Laminar cooling adopts a three-stage cooling process of a front-stage concentrated cooling zone, an air cooling zone, and a fine-tuning water cooling zone, which together constitute the cooling process path of the hot-rolled steel strip. By precisely controlling the cooling method and temperature of each area, the organizational structure, mechanical properties, and surface quality of the steel strip can be optimized, thereby reducing the occurrence of defects such as flattening. At the same time, this cooling process path ensures that the organizational properties meet the requirements, and enables the steel strip to undergo a phase change in advance before coiling, thereby reducing the interlayer displacement after coiling and significantly improving the flattening problem. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present application, and together with the description, are used to explain the principles of the present application.

[0027] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0028] Figure 1 A schematic flow chart of a method for preventing hot-rolled steel strip from flattening provided in an embodiment of the present application;

[0029] Figure 2 A schematic diagram of a cooling path for preventing a hot-rolled steel strip from flattening provided in an embodiment of the present application;

[0030] Reference numerals:

[0031] 1-hot rolled strip, 2-finishing mill, 3-output roller, 4-finishing exit pyrometer, 5-front section cooling header, 6-intermediate point pyrometer, 7-fine adjustment front pyrometer, 8-fine adjustment section header, 9-coil inlet pyrometer, 10-coiler. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0033] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have 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., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0034] In addition, in the description of the specification of the present application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second", etc. are only used 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. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" means one or more, and "plurality" means two or more. "At least one", "the following at least one item (items)" or similar expressions refer to any combination of these items, including any combination of single items (items) or plural items (items). For example, "at least one of a, b, or c", or "at least one of a, b, and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one-to-one with the proportional numbers in the proportional formula 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.

[0035] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0036] Figure 1 A schematic flow chart of a method for preventing hot-rolled steel strip from flattening provided in an embodiment of the present application.

[0037] like Figure 1 As shown, the present application provides a method for preventing hot rolled steel strip from flattening, the method comprising:

[0038] S1. At a set final rolling outlet temperature, the rough-rolled sheet is subjected to finish rolling to obtain a hot-rolled strip;

[0039] The finishing process not only improves the shape and dimensional accuracy of the strip, but also provides favorable conditions for phase transformation in the subsequent cooling process by refining the austenite grains and homogenizing the structure, thereby helping to reduce the occurrence of flat coil defects.

[0040] In some embodiments, the set final rolling temperature is 820°C to 920°C.

[0041] The final rolling temperature of the finishing rolling is limited to 820℃~920℃, which can ensure that the strip has an appropriate austenite grain size before finishing rolling. Too high a final rolling temperature may cause the grains to be too coarse, while too low a temperature may cause the grains to be too fine, which is not conducive to obtaining ideal structural properties. Exemplarily, the final rolling temperature of the finishing rolling can be 820℃, 840℃, 860℃, 880℃, 900℃, 920℃, etc.

[0042] In some embodiments, the inlet temperature of the finish rolling is 1030°C to 1130°C.

[0043] Limiting the inlet temperature of the finishing rolling to 1030°C to 1130°C can ensure that the strip has sufficient plasticity and deformation capacity, which is conducive to obtaining a good rolling effect and surface quality. At the same time, the appropriate inlet temperature also helps to reduce energy consumption and wear during the rolling process, improve production efficiency and product quality. Exemplarily, the inlet temperature of the finishing rolling can be 1030°C, 1040°C, 1050°C, 1060°C, 1070°C, etc.

[0044] S2, conveying the hot-rolled steel strip to a cooling line having a front-stage concentrated cooling zone, an air cooling zone, and a fine-adjustment water cooling zone;

[0045] The front concentrated cooling zone is located on the output roller after the finishing mill and is the first stage of hot-rolled strip cooling. It uses laminar cooling and accurately controls the temperature of the strip at the midpoint (MT1) by adjusting the opening and closing of the cooling manifold and the amount of cooling water.

[0046] The air cooling zone is located after the front concentrated cooling zone and before the fine-tuning water cooling zone. Its main function is to allow the strip to cool naturally on the conveyor rollers, allowing the austenite to fully undergo phase transformation.

[0047] The fine-tuning water cooling zone is located after the air cooling zone and is the final stage of hot-rolled strip cooling. Its main function is to cool the strip to the set final cooling temperature (CT) by accurately controlling the cooling water volume and temperature to improve its microstructure, properties and surface quality.

[0048] The front concentrated cooling zone, air cooling zone and fine-tuning water cooling zone together constitute the cooling process path of hot-rolled strip. By precisely controlling the cooling method and temperature of each zone, the organizational structure, mechanical properties and surface quality of the strip can be optimized, thereby reducing the occurrence of defects such as flat coils. This cooling process path ensures that the organizational properties meet the requirements, enables the strip to undergo phase change in advance before coiling, reduces interlayer displacement after coiling, and significantly improves the flat coil problem.

[0049] S3, in the front concentrated cooling zone, the hot rolled steel strip is subjected to front concentrated cooling, and the midpoint temperature of the cooling line is controlled to be a set midpoint temperature to control the grain size of the steel strip; the set midpoint temperature is greater than the bainite starting phase transformation temperature;

[0050] In the front concentrated cooling zone, the hot rolled strip is cooled quickly to control its temperature and grain size. Setting the midpoint temperature (MT1) of the cooling line above the bainite start transformation temperature (BS) can avoid premature bainite or martensite transformation of the strip, thereby reducing the generation of brittle structure.

[0051] In some embodiments, the midpoint temperature of the cooling line is the temperature of the hot-rolled strip at the middle temperature measurement point in the length direction of the cooling line, and the set midpoint temperature satisfies the following relationship:

[0052] MT1=BS+(10~50)℃

[0053] Wherein, MT1 represents the set midpoint temperature, and BS represents the bainite starting phase transformation temperature.

[0054] Controlling the midpoint temperature to be 10 to 50°C higher than the bainite start phase transformation temperature (BS) can prevent the strip from undergoing bainite or martensite transformation prematurely during water cooling, which usually leads to the formation of brittle structures. Keeping the midpoint temperature within this range is conducive to the transformation of austenite to more favorable structures such as ferrite and pearlite during subsequent air cooling, thereby improving the organizational properties and mechanical properties of the strip. At the same time, the temperature is as low as possible to make the subsequent phase transformation structure finer and ensure the spheroidization rate after cold rolling annealing. Exemplarily, MT1 can be BS+10°C, BS+20°C, BS+30°C, BS+40°C, BS+50°C, etc.

[0055] S4, in the air cooling zone, air cooling the hot rolled steel strip after the concentrated cooling in the previous stage to perform phase transformation at a lower cooling rate;

[0056] In the air cooling zone, air cooling is performed on the conveyor rollers to allow the strip to fully undergo phase change. The air cooling process helps to balance the temperature and structure differences inside the strip, reduce the volume expansion differences caused by uneven phase change, and thus reduce the risk of flat coils.

[0057] In some embodiments, the final cooling temperature of the air cooling is 550°C to 650°C.

[0058] The final cooling temperature of air cooling is limited to 550℃~650℃, which is conducive to the transformation of austenite into a more favorable structure, while avoiding the adverse effects of too high or too low temperature on the structure performance. Temperature control during air cooling also helps to reduce stress concentration and deformation inside the strip, thereby improving the quality and stability of the product. Exemplarily, the final cooling temperature of air cooling can be 550℃, 570℃, 590℃, 600℃, 620℃, 640℃, 650℃, etc.

[0059] S5, in the fine-adjustment section water cooling zone, subjecting the hot-rolled steel strip after air cooling to fine-adjustment section water cooling, and controlling the final cooling temperature of the fine-adjustment section water cooling to the set final cooling temperature, so as to improve the microstructure and properties of the hot-rolled steel strip; and

[0060] In the water cooling area of ​​the fine adjustment section, the hot rolled strip after air cooling is cooled to the set final cooling temperature (CT) by accurately controlling the water volume of the cooling manifold. This step not only further improves the organizational properties of the strip, but also avoids deformation and flat coiling caused by uneven temperature through precise temperature control.

[0061] In some embodiments, the final cooling temperature is set to be 500°C to 650°C.

[0062] The final cooling temperature of the water cooling zone of the fine-tuning section is limited to 500℃~650℃, which can ensure that the strip obtains the ideal structure and performance, such as good strength, toughness and plasticity. At the same time, the formability and processing performance of the strip can be further improved, and the occurrence of defects such as flat coils can be reduced. Exemplarily, the final cooling temperature of the water cooling zone of the fine-tuning section is 500℃, 550℃, 570℃, 590℃, 600℃, 620℃, 640℃, 650℃, etc.

[0063] In some implementations, the cooling manifold water volume of the fine-adjustment section water cooling is determined according to the final cooling temperature of the air cooling, the set final cooling temperature of the fine-adjustment section water cooling, and the latent heat of phase change.

[0064] The control of the water volume in the cooling manifold is determined based on the strip temperature after air cooling (MT2) and the required final cooling temperature (CT). By accurately controlling the water volume, it is possible to ensure that the strip obtains an ideal cooling effect during the water cooling process in the fine-tuning section, thereby further improving the structural properties and mechanical properties of the strip. At the same time, the control of the water volume also needs to consider the influence of the latent heat of phase change to ensure that the strip can undergo a uniform and stable phase change during the cooling process.

[0065] S6, coiling the hot-rolled steel strip after water cooling in the fine-adjusting section to obtain a hot-rolled coil.

[0066] In some embodiments, the method for preparing the rough rolled plate comprises:

[0067] The slab with a set chemical composition is heated and roughly rolled to obtain the rough-rolled plate.

[0068] In some embodiments, the rough rolling adopts a 3+3 mode.

[0069] In some embodiments, the heating time is 140 min to 300 min, and the tapping temperature is 1160°C to 1260°C.

[0070] In summary, the method for preventing hot rolled steel strip from flattening provided in the embodiment of the present application has the following advantages:

[0071] (1) Optimization of finishing rolling process: By setting reasonable final rolling temperature and inlet temperature (final rolling temperature is 820℃~920℃, inlet temperature is 1030℃~1130℃), the strip is ensured to have appropriate austenite grain size and deformation capacity during the finishing rolling process. The finishing rolling process not only improves the shape and dimensional accuracy of the strip, but also provides favorable conditions for phase transformation in the subsequent cooling process by refining the austenite grains and homogenizing the structure, which helps to reduce the occurrence of flat coil defects.

[0072] (2) Precise control of cooling process: The front concentrated cooling zone, air cooling zone and fine-tuning water cooling zone together constitute the cooling process path of the hot-rolled strip. By precisely controlling the cooling method and temperature of each zone, the microstructure and mechanical properties of the strip are optimized. In the front concentrated cooling zone, by rapidly cooling and controlling the midpoint temperature to be 10 to 50°C higher than the bainite phase transformation temperature, the premature brittle phase transformation of the strip is avoided and the formation of brittle phase structure is reduced. The air cooling zone allows the austenite to fully transform, balances the temperature and microstructure differences inside the strip, and reduces the risk of flat coiling. The fine-tuning water cooling zone cools the strip to the set final cooling temperature (500°C to 650°C) by precisely controlling the cooling water volume and temperature, further improving the microstructure and mechanical properties of the strip.

[0073] (3) Reducing flat coil defects: This method significantly reduces the interlayer displacement of hot-rolled strip during coiling by optimizing the finishing process and accurately controlling the cooling process, thereby reducing the incidence of flat coil defects. Temperature control during the cooling process helps reduce stress concentration and deformation inside the strip, improving product quality and stability.

[0074] (4) Improve production efficiency and product quality: By precisely controlling various process parameters and conditions, this method ensures that the hot-rolled strip undergoes phase transformation in advance before coiling, reducing deformation and defects during subsequent processing, thereby improving production efficiency and product quality. At the same time, this method also takes into account the influence of latent heat of phase transformation, ensuring that the strip undergoes phase transformation evenly and stably during the cooling process, further improving product stability and reliability.

[0075] (5) Flexibility and applicability: This method is not only applicable to the production of medium and high carbon steel products, but also can adjust process parameters according to specific needs to adapt to the production of slabs with different chemical compositions and specifications. At the same time, by adjusting parameters such as rough rolling mode, heating time and steel tapping temperature, the production process and product quality can be further optimized.

[0076] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to industry standards. If there is no corresponding industry standard, then the conditions recommended by the manufacturer are followed.

[0077] Example 1

[0078] This embodiment provides a method for preventing hot rolled steel strip from flattening, the method comprising the following steps:

[0079] Step 1: Smelt and continuously cast the molten steel to obtain a slab; the chemical composition of the slab is as follows: C: 0.75%, Si: 0.18%, Mn: 0.65%, Cr: 0.25%, S: 0.009%, P: 0.08%, Al: 0.01%, and the rest is iron and residue. The thickness of the slab is 237 mm;

[0080] Step 2, the slab is heated, rough rolled and finished rolled in a conventional hot rolling production line to obtain hot rolled strip 1; the heating time of the heating furnace is 140min, the steel tapping temperature is 1180°C, the rough rolling mode is 3+3, the finishing rolling inlet temperature is 1030°C, the final rolling temperature FDT is 820°C, and the thickness of the rolled product is 2.1mm;

[0081] Figure 2 A schematic diagram of a cooling path for preventing flat coils of hot-rolled steel strip provided in an embodiment of the present application.

[0082] Step 3: Figure 2 As shown, the hot-rolled steel strip 1 passes through the finishing mill 2 and enters the coiler 10 through the output roller 3. There is a temperature measuring device 4 after the finishing mill 2, and the temperature of the steel strip at this location is required to be 820°C; the laminar cooling adopts a three-stage cooling process. First, the cooling manifold device 5 is used on the conveying roller for front-stage centralized cooling (water cooling), and there is a temperature measuring point 6 in the middle of the output roller, and the temperature of MT1 is controlled to be 525°C (the bainite phase transformation point is measured to be 510°C); secondly, the steel strip 1 is air-cooled between 6 and 7 and fully undergoes phase transformation, and MT2 is 570°C after phase transformation; finally, the fine-tuning water cooling is carried out at the cooling manifold device 8, and the final cooling temperature CT of the fine-tuning water cooling is controlled to be 500°C.

[0083] This production was 1,000 tons, and the final return rate was 0% due to flat coil defects.

[0084] Example 2

[0085] This embodiment is modified as follows based on the disclosure of Embodiment 1:

[0086] The heating time of the heating furnace is 180min, the steel-out temperature is 1200℃, the rough rolling mode is 3+3, the finishing rolling inlet temperature is 1050℃, and the final rolling temperature is 860℃; the laminar cooling adopts a three-stage cooling process, the first stage is the front stage concentrated cooling, and the MT1 is controlled at 540℃; the second stage is air cooling, and the MT2 is 600℃ after phase change; the third stage is fine-tuning water cooling, and the CT is controlled at 550℃.

[0087] This production was 1,000 tons, and the final return rate was 0% due to flat coil defects.

[0088] Example 3

[0089] This embodiment is modified as follows based on the disclosure of Embodiment 1:

[0090] The heating time of the heating furnace is 260min, the steel-out temperature is 1260℃, the rough rolling mode is 3+3, the finishing rolling inlet temperature is 1120℃, and the final rolling temperature is 920℃; the laminar cooling adopts a three-stage cooling process, the first stage is the front stage concentrated cooling, and the MT1 is controlled at 560℃; the second stage is air cooling, and the MT2 is 640℃ after phase change; the third stage is fine-tuning water cooling, and the CT is controlled at 630℃.

[0091] This production was 1,000 tons, and the final rebate rate was 2% due to flat coil defects.

[0092] Comparative Example 1

[0093] This comparative example is modified as follows based on the disclosure of Example 1:

[0094] The heating time of the heating furnace is 160min, the steel tapping temperature is 1200℃, the rough rolling mode is 3+3, the finishing rolling inlet temperature is 1050℃, and the final rolling temperature is 830℃; the laminar cooling adopts front-stage centralized cooling, that is, it enters the water cooling zone after finishing rolling, and does not control the temperature of the middle section and the temperature before fine-tuning water cooling. Only the final water cooling temperature is controlled to 550℃.

[0095] This production was 800 tons, and the final return rate was 60% due to flat coil defects.

[0096] In addition, one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages:

[0097] In the embodiment of the present application, the three-stage cold process is a special post-rolling cooling process, which achieves the matching of phase change and performance by controlling the midpoint of the strip in the cooling area and the final cooling temperature, and can significantly improve the flat coil problem in the production process of medium and high carbon steel.

[0098] In the embodiments of the present application, by controlling the cooling process path in a conventional hot rolling production line, the strip is phase-changed in advance before coiling while ensuring that the microstructure and performance meet the requirements, thereby reducing the interlayer displacement after coiling and significantly improving the flat coil problem.

[0099] In the embodiment of the present application, the MT is controlled to make the strip undergo phase change on the output roller table, thereby improving the flat coil.

[0100] In the embodiments of the present application, the final cooling temperature is controlled by controlling the latent heat of phase change and fine-tuning the amount of water to improve the microstructure and properties of the strip.

[0101] In the embodiments of the present application, the return rate due to flat coil defects is ≤2%, which is much lower than that of conventional cooling processes.

[0102] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for preventing hot rolled strip from flattening, the method comprising: At a set final rolling exit temperature, the rough rolled sheet is subjected to finish rolling to obtain a hot rolled steel strip; The hot-rolled steel strip is conveyed to a cooling line having a front-stage concentrated cooling zone, an air cooling zone and a fine-adjustment water cooling zone; In the front concentrated cooling zone, the hot rolled steel strip is subjected to front concentrated cooling, and the midpoint temperature of the cooling line is controlled to be a set midpoint temperature to control the grain size of the steel strip; the set midpoint temperature is greater than the bainite starting phase transformation temperature; In the air cooling zone, the hot rolled steel strip is air cooled after the concentrated cooling in the previous stage to perform phase transformation at a lower cooling rate; In the fine-adjustment section water cooling zone, the hot-rolled steel strip after air cooling is subjected to fine-adjustment section water cooling, and the final cooling temperature of the fine-adjustment section water cooling is controlled to be a set final cooling temperature, so as to improve the microstructure and properties of the hot-rolled steel strip; and The hot-rolled steel strip after water cooling in the fine-tuning section is coiled to obtain a hot-rolled coil.

2. The method according to claim 1, characterized in that The final rolling temperature is set to 820°C to 920°C.

3. The method according to claim 1, characterized in that: The midpoint temperature of the cooling line is the temperature of the hot-rolled strip at the middle temperature measuring point in the length direction of the cooling line. The set midpoint temperature satisfies the following relationship: MT1=BS+(10~50)℃ Wherein, MT1 represents the set midpoint temperature, and BS represents the bainite starting phase transformation temperature.

4. The method according to claim 1, characterized in that: The final cooling temperature is set to be 500°C to 650°C.

5. The method according to claim 1, characterized in that The water volume of the cooling manifold of the fine-adjustment section water cooling is determined according to the final cooling temperature of the air cooling, the set final cooling temperature of the fine-adjustment section water cooling, and the latent heat of phase change.

6. The method according to claim 1, characterized in that The final cooling temperature of the air cooling is 550°C to 650°C.

7. The method according to claim 1, characterized in that The inlet temperature of the finish rolling is 1030°C to 1130°C.

8. The method according to claim 1, characterized in that The method for preparing the rough rolled plate comprises: The slab with a set chemical composition is heated and roughly rolled to obtain the rough-rolled plate.

9. The method according to claim 8, characterized in that The rough rolling adopts 3+3 mode.

10. The method according to claim 8, characterized in that The heating time of the heating is 140 minutes to 300 minutes, and the steel tapping temperature of the heating is 1160°C to 1260°C.

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