A method of producing a low-stress hot-rolled steel coil

Abstract

The application discloses a low-stress hot-rolled steel coil production method, which comprises the following steps: controlling the rigidity difference between the two sides of a rolling mill within a set range and setting the steel plate crown in the rolling process to realize the stability of the manufacturing process and the original plate shape of the steel plate; determining the crown of the final state steel plate according to the middle part temperature of the steel plate, the temperature of the transmission side edge of the steel plate and the temperature of the operation side edge of the steel plate in the steel strip rolling process; and the rolling mill cooling system comprises side spray cooling valves arranged at the two side edges of the steel strip, so that the pressure flow of the side spray cooling valves reaches the condition that the surface cooling water of the steel plate does not gather at the side edges, and the cooling intensity of the side edge of the steel strip and the middle part of the steel strip is uniform. The application can meet the low-stress and high-plate-shape requirements of the steel coil, and is especially suitable for the treatment of the steel strip with the thickness of 3-8 mm of the steel coil.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a method for producing low-stress hot-rolled steel coils. Background Technology

[0002] Strip shape is a crucial and measurable macroscopic quality indicator for hot-rolled strip steel, while internal stress is a difficult-to-monitor microscopic technical indicator. Strip shape essentially reflects the distribution of residual internal stress. When residual internal stress exceeds a certain critical value, it directly leads to strip deformation. Uneven distribution of residual internal stress can also cause potential deformation during subsequent processing and application. Therefore, improving the strip shape quality and reducing the original internal stress of hot-rolled strip steel through technological means is crucial for the overall quality of hot-rolled strip steel. The process flow of a hot continuous rolling mill generally includes heating, roughing, finishing, and coiling. The quality of hot-rolled strip shape, internal stress, and each step of the hot rolling process are related. Unlike single-rolled hot-rolled strip steel produced in medium and heavy plate production lines, which has its own technological characteristics, hot-rolled steel coils generally have higher internal stress, especially due to three problems: (1) rolling and cooling internal stress caused by uneven deformation and uneven cooling during the strip rolling process; (2) uneven curvature distribution along the length direction during the coiling process, resulting in uneven stress from the inner to the outer ring of the strip and on the upper and lower surfaces during the coiling process; (3) various uneven residual stresses will be formed inside and outside the coil after the hot-rolled strip is coiled due to uneven cooling. This patent proposes a low-stress production method for thin-gauge hot-rolled steel coils by controlling the internal stress of the strip shape through the strip rolling, cooling and heat preservation processes. Summary of the Invention

[0003] The purpose of this invention is to provide a low-stress hot-rolled steel coil production method that can meet the requirements for low stress and high plate shape of steel coils, and is especially suitable for processing steel coils with a strip thickness of 3~8mm.

[0004] The technical solution adopted in this invention is:

[0005] A method for producing low-stress hot-rolled steel coils, comprising:

[0006] By controlling the stiffness difference between the two sides of the rolling mill and setting the convexity of the steel plate during the rolling process, the stability of the manufacturing process and the original shape of the steel plate are achieved, and the internal stress of rolling is reduced.

[0007] The convexity of the steel plate is determined based on the temperature of the middle part of the steel plate, the temperature of the transmission side of the steel plate, and the temperature of the operation side of the steel plate during the strip rolling process.

[0008] The mill cooling system includes side spray cooling valves located on both sides of the strip. The nozzles connected to the side spray cooling valves are angled to avoid the overcooled parts of the strip edges, and the pressure and flow rate of the side spray cooling valves are sufficient to prevent cooling water from accumulating on the steel plate surface at the edges, thus ensuring uniform cooling intensity at the edges and center of the strip.

[0009] Preferably, the final convexity of the steel plate is Y = K*[T] 中 -1 / 2(T 传动侧 +T 操作侧 )]

[0010] Where K is the high-temperature plasticity constant of the rolled steel, and T 中 T represents the temperature at the center of the steel plate. 传动侧 Temperature of the side of the steel plate transmission, T 操作侧 This refers to the temperature of the operating side of the steel plate.

[0011] Preferably, the manufacturing process stability and the original shape of the steel plate are controlled by the stiffness difference between the two sides of the rolling mill, thereby reducing the internal stress during rolling. The stiffness difference between the two sides of the rolling mill (drive side and working side) is controlled within 30 tons / mm.

[0012] The cooling system includes an ultra-fast cooling unit, a dense cooling unit, and a compensating cooling unit arranged sequentially along the steel strip conveying direction of the rolling mill. A cross-sectional temperature detector is installed after the ultra-fast cooling unit. The cross-sectional temperature detector is used to collect steel strip cross-sectional temperature data in real time. The cross-sectional temperature detector is connected to the ultra-fast cooling unit, the dense cooling unit, and the compensating cooling unit respectively.

[0013] Preferably, the ultra-fast cooling unit is arranged in the first 1 / 3 of the rolling mill conveyor, the dense cooling unit is arranged in the middle 1 / 3 of the rolling mill conveyor, and the compensating cooling unit is arranged in the last 1 / 3 of the rolling mill conveyor. A dedicated cross-sectional temperature monitoring instrument is set after the ultra-fast cooling unit in the first 1 / 3 of the cooling rolling direction to collect cross-sectional temperature data in real time. For cross-sectional unevenness data, the rear 1 / 3 compensating cooling system is designed with rear-stage compensating cooling control. After the steel strip is cooled, the cross-sectional cooling effect is fed back to the ultra-fast cooling device by the terminal cross-sectional temperature monitoring instrument to realize the correction of the cooling parameters of the next steel strip. The cooling capacity of the ultra-fast cooling unit, the dense cooling unit, and the compensating cooling unit decreases sequentially.

[0014] Preferably, the water spray ratio of the upper and lower strip cooling systems is set to 1:1 to 1:1.5, with asymmetric water ratio for uniform cooling in the thickness direction. This ratio should be adjusted for different thicknesses of wide strip steel plates, and horizontal fine-tuning should be performed in combination with the uniformity of the steel plate's microstructure and properties to determine the final optimized water ratio.

[0015] Preferably, to address the issue of stress differences between the upper and lower surfaces caused by different curvatures along the length of the steel coil, a reverse flattening or straightening process is adopted, which allows the steel coil curvature to bend in the opposite direction, thereby further reducing the tension along the length.

[0016] Preferably, to address the residual stress difference in the width direction of the steel coil, the entire steel coil after coiling is insulated. The steel coil is then stored by stacking it using its own inherent temperature or annealed (500~650℃) by the relatively uniform temperature of the insulation wall or indoor environment. This stacking and sealing insulation measure can effectively solve the internal and external stress difference of the steel strip.

[0017] Preferably, the annealing temperature for the steel coil is 500~650℃.

[0018] Preferably, the steel strip thickness of the steel coil is 3~8mm.

[0019] The beneficial effects of this invention are:

[0020] 1. By controlling the crown and temperature of the steel plate during the rolling and coiling process, and combining the internal stress characteristics of the rolled steel coil, the requirements for low stress and high plate shape of the steel coil can be met, especially suitable for the processing of steel coils with a strip thickness of 3~8mm.

[0021] 2. In conjunction with the steel plate crown and temperature during the rolling and coiling process, the mill precision and coil insulation are matched and controlled. Unlike the stress elimination method of coiled flat plates through heat treatment, this invention provides a heat treatment-free process. It reduces the residual stress of the strip steel through small crown uniform plastic deformation, stable rolling of the mill, uniform surface cooling, low tension coiling, and self-tempering treatment of the coil. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection. They can refer to a mechanical connection or an electrical connection. They can refer to a direct connection or an indirect connection through an intermediate medium, and they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances.

[0024] A method for producing low-stress hot-rolled steel coils includes: controlling the stiffness difference between the two sides of the rolling mill and setting the convexity of the steel plate during the rolling process to achieve stability in the manufacturing process and the original shape of the steel plate, thereby reducing rolling internal stress;

[0025] The convexity of the steel plate is determined based on the temperature of the middle part of the steel plate, the temperature of the transmission side of the steel plate, and the temperature of the operation side of the steel plate during the strip rolling process.

[0026] The mill cooling system includes side spray cooling valves located on both sides of the strip. The nozzles connected to the side spray cooling valves are angled to avoid the overcooled parts of the strip (overcooled parts refer to the parts with a large temperature drop at the edge, generally within 100mm of the edge). The pressure and flow rate of the side spray cooling valves are such that cooling water does not accumulate on the surface of the steel plate at the edge, and the cooling intensity of the strip edge and the middle of the strip is uniform.

[0027] The thickness of the strip in the steel coil is 3~8mm.

[0028] Furthermore, the final convexity of the steel plate is Y = K*[T] 中 -1 / 2(T 传动侧 +T 操作侧 )]

[0029] Where K is the high-temperature plasticity constant of the rolled steel, and T 中 T represents the temperature at the center of the steel plate. 传动侧 Temperature of the side of the steel plate transmission, T 操作侧 The temperature of the operating side of the steel plate is used to determine the convexity of the steel plate based on the temperature gradient in the width direction during the strip rolling process.

[0030] Furthermore, the temperature at the center of the steel plate specifically refers to the temperature at the center of the steel plate during the rolling process; the temperature at the edge of the steel plate on the drive side specifically refers to the temperature at the edge of the strip near the drive side of the rolling mill during the rolling process; and the temperature at the edge of the steel plate on the operating side specifically refers to the temperature at the edge of the strip near the operating side of the rolling mill during the rolling process.

[0031] Furthermore, precise control of the plastic deformation of the slab during the rolling process, especially the finishing rolling process, is required. To ensure the stability of the strip rolling process, a certain degree of crowning is designed, which to some extent exacerbates the unevenness of residual internal stress. Due to the crowning, the edge thickness of the strip is less than that of the middle. After layer-by-layer accumulation in the later stages of coiling, it will exhibit a deformation in the width direction that is higher in the middle and bends downwards on both sides. When this bending deformation exceeds a certain level, the strip will undergo plastic deformation, forming transverse warping, which can easily lead to stress caused by uneven plastic deformation during coiling.

[0032] Furthermore, in actual production, the phenomena of side bends, cambers, and large lateral movement of hot-rolled strips at the head and tail, as well as head folds in thin-gauge steel strips, can lead to defects in the coil shape, such as head towering and overflow edges. These defects are often accompanied by significant internal stress, resulting in poor original rolled strip shape. Therefore, it is necessary to improve the coil shape quality through process control during rolling. Specifically, this involves identifying the correlation between the camber of the rolled strip shape and the stiffness, pressure difference, and precision of equipment such as AGC cylinders in the finishing mill. During the rolling process, under the action of rolling force, the rolled piece undergoes plastic deformation, changing its thickness and cross-sectional shape. The reaction force of the rolled piece causes a series of components in the working stand, such as rolls, roll bearings, bearing housings, pads, pressing cylinders, and arches, to undergo corresponding elastic deformation. The sum of the elastic deformations of these stressed components is usually called the mill bounce value. The stiffness of a rolling mill refers to its ability to resist elastic deformation caused by rolling pressure; it is also known as the mill modulus or mill modulus, and has a significant impact on mill adjustment and workpiece dimensional accuracy. The difference in elastic deformation between the two sides of the rolling mill is a major factor contributing to wedge-shaped bending, sickle-shaped bending, and rolling instability during the rolling process. When the stiffness difference between the two sides of the rolling mill (drive side and working side) exceeds 30 tons / mm, the mill will exhibit significant rolling instability and wedge-shaped loss of control. Therefore, in actual production, the stability of the manufacturing process and the original shape of the steel plate can be controlled by adjusting the stiffness difference between the two sides of the rolling mill, thereby reducing rolling internal stress.

[0033] The cooling system includes an ultra-fast cooling unit, a dense cooling unit, and a compensating cooling unit arranged sequentially along the steel strip conveying direction of the rolling mill. A cross-sectional temperature detector is installed after the ultra-fast cooling unit. The cross-sectional temperature detector is used to collect steel strip cross-sectional temperature data in real time. The cross-sectional temperature detector is connected to the ultra-fast cooling unit, the dense cooling unit, and the compensating cooling unit respectively.

[0034] Furthermore, uneven cooling in the width direction: Generally speaking, the edges of the steel plate cool faster and undergo phase transformation earlier than the middle part, entering the expansion stage, while the middle part has not yet undergone phase transformation and is still in the contraction process. As the temperature continues to decrease, this contradiction gradually intensifies, resulting in a certain stress between the edges and the middle part of the steel plate. When this stress increases to the point that it exceeds the deformation resistance of the steel plate, edge waviness defects will appear on the steel plate. Solution: (1) Side spray cooling valve angle and flow control: Set up dedicated side spray cooling valves on both sides, and make the nozzle angle avoid the overcooled parts on the side, and make the water valve pressure and flow rate of the side spray reach the point where the cooling water on the steel plate surface does not accumulate on the side, so that the cooling intensity of the side and the middle is uniform; (2) The cooling system is divided into the head 1 / 3 ultra-fast cooling, the middle 1 / 3 dense cooling, and the rear 1 / 3 compensation cooling. Set up a dedicated cross-section temperature monitoring instrument after the head 1 / 3 ultra-fast cooling in the cooling rolling direction, and collect the cross-section temperature data in real time. For the cross-section uneven data, the rear 1 / 3 compensation cooling system is designed with the rear compensation cooling control. After the steel strip is cooled, the cross-section cooling effect is fed back to the ultra-fast cooling device by the terminal cross-section temperature monitoring instrument, so as to realize the correction of the cooling parameters of the next steel strip. The cooling capacity of the ultra-fast cooling unit, the dense cooling unit and the compensation cooling unit decreases in sequence.

[0035] Furthermore, uneven cooling in the thickness direction occurs: during the cooling process, the upper surface of the steel plate cools faster than the lower surface, entering the phase transformation zone earlier and expanding in volume, forming a "turtleback" shape. When the lower surface undergoes phase transformation and expands in volume subsequently, the metal units are subjected to compressive stress, making it impossible to eliminate the arching. When this residual stress is large, it may form L-shaped, C-shaped, or four-corner warping during finishing and shearing. Solution: Achieving symmetrical cooling in the thickness direction mainly involves ensuring that the upper and lower surfaces of the steel plate receive the same cooling effect during the cooling process. For different steel grades and thicknesses, the water spray ratio is set to an asymmetric water ratio of 1:1 to 1:1.5 for uniform cooling in the thickness direction. This ratio should be adjusted for different thicknesses of wide steel plates, and horizontal fine-tuning should be performed in conjunction with the uniformity of the steel plate's microstructure and properties to determine the final optimized water ratio.

[0036] Furthermore, during the coiling process, the tension control of the inner and outer rings and the upper and lower surfaces reveals that the residual stress value increases with the yield point and original curvature of the steel. The degree of warping varies with the strength and original curvature of the steel plate; higher strength and greater original curvature result in more severe warping. Moreover, the warping direction of each strip after slitting is likely to be the same, which aligns with actual warping conditions. Analysis indicates that to solve the warping problem of hot-rolled steel plates after slitting, it is essential to first eliminate the stress difference between the upper and lower surfaces caused by the curvature along the length of the strip, and then homogenize the residual stress in the width direction. Conventional straightening can produce a completely straight steel plate, but its effect on preventing warping after slitting is not significant. To address the issue of stress differences between the upper and lower surfaces caused by varying curvature along the length, a reverse flattening or straightening process is employed. This allows the steel coil's curvature to bend in the opposite direction, further reducing tension along the length. To address residual stress differences along the width, the steel coil is insulated as a whole. The coil is then annealed (500~650℃) using its inherent temperature during stacking or by the relatively uniform temperature of the insulation wall or indoor environment. This stacking and sealing insulation approach effectively resolves the internal and external stress differences in the strip steel.

[0037] To better understand the present invention, the following description, in conjunction with the embodiments in Table 1, further clarifies the content of the present invention, but the present invention is not limited to the embodiments below. Embodiments 1-8 and Comparative Examples 1-4: A high-strength ship plate AH36 was produced by hot continuous rolling of 230mm thick slabs on a 2250mm hot continuous rolling mill using a controlled rolling and controlled cooling process, and the slabs were rolled into 6mm hot-rolled steel plates.

[0038] Table 1 Examples

[0039] serial number Rolled steel crown μm Difference in load stiffness of precision rolling (t / mm) Cooling uniformity (°C) Slow-cooling insulation temperature (°C) Average internal stress of steel plate (MPa) Example 1 10 15 10 650 -16 Example 2 18 20 5 630 -16 Example 3 25 17 15 610 -47 Example 4 28 20 11 600 -38 Example 5 32 25 18 580 -50 Example 6 35 30 13 550 -72 Example 7 40 35 20 500 -74 Example 8 25 10 14 580 -60 Comparative Example 1 80 39 25 400 -148 Comparative Example 2 90 31 19 430 -133 Comparative Example 3 45 70 40 510 -157 Comparative Example 4 40 38 24 No insulation or slow cooling measures -180

[0040] For the stiffness difference under multi-stand finishing rolling load, the maximum stiffness difference between stands is used as the recording parameter. For the uniformity of the rolled steel plate in the thickness direction, the top and bottom water ratio is used for control, and the width temperature difference is used to evaluate the cooling uniformity in the width direction of the rolled steel plate. The average temperature is used for slow cooling and heat preservation. The average internal stress of the steel plate is calculated by performing multi-point stress detection on the entire width of the plate using the residual stress method after the steel coil is uncoiled using X-ray diffraction, and then calculating the average value.

[0041] As can be seen from Examples 1-8 and Comparative Examples 1-4, the residual stress of the strip steel can be reduced to ≤74MPa by means of small crown uniform plastic deformation rolling, precision control of finishing rolling load, uniform surface cooling, low tension coiling, and self-tempering treatment of steel coils. However, by using the comparative example process, the residual stress is ≥100MPa.

[0042] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0043] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method of producing a low-stress hot-rolled steel coil, characterized by: The application comprises the following steps: The stability of the manufacturing process and the original plate shape of the steel plate are realized by controlling the rigidity difference between the two sides of the rolling mill within a set range and setting the steel plate crown during the rolling process; The steel plate crown is determined according to the temperature of the middle part of the steel plate, the temperature of the driving side edge of the steel plate and the temperature of the operating side edge of the steel plate during the strip rolling process; The cooling system of the rolling mill comprises side spray cooling valves arranged at the two side edges of the strip, so that the pressure flow of the side spray cooling valves reaches the condition that the cooling water on the surface of the steel plate does not gather at the side edges, and the cooling intensity of the side edges of the strip and the middle part of the strip is uniform. Steel sheet camber Y = K * [T 中 -1 / 2(T 传动侧 +T 操作侧 )] wherein K is a high temperature plasticity constant of the rolled steel material, T 中 is a temperature at a middle portion of the steel sheet, T 传动侧 is a temperature at a driving side edge portion of the steel sheet, T 操作侧 is a temperature at an operating side edge portion of the steel sheet.

2. The method of producing a low-stress hot rolled steel coil of claim 1, wherein: The rigidity difference between the two sides of the rolling mill is controlled within 30 tons / mm.

3. The method of producing a low-stress hot rolled steel coil of claim 1 wherein: The cooling system comprises an ultra-fast cooling unit, an encryption cooling unit and a compensation cooling unit arranged in sequence along the conveying direction of the steel strip of the rolling mill, and a cross-section temperature detector is arranged behind the ultra-fast cooling unit, which is used for real-time acquisition of the cross-section temperature data of the steel strip and is connected with the ultra-fast cooling unit, the encryption cooling unit and the compensation cooling unit.

4. The method of producing a low-stress hot rolled steel coil of claim 1 wherein: The water spraying amount of the upper side cooling system and the lower side cooling system of the strip is set to 1:1~1:1.

5.

5. The method of producing a low-stress hot rolled steel coil of claim 1 wherein: The reverse flattening or straightening process is adopted to reduce the length direction tension of the steel coil through reverse bending.

6. The method of producing a low-stress hot rolled steel coil of claim 1 wherein: The whole steel coil after coiling is heat preserved, and the steel coil is stacked and stored by using the self temperature of the steel coil or is annealed by using the relatively uniform temperature in the room through the heat preservation wall.

7. The method of producing a low stress hot rolled steel coil as claimed in claim 6 wherein: The annealing temperature of the steel coil is 500~650℃.

8. The method of producing a low-stress hot rolled steel coil of claim 1 wherein: The thickness of the steel strip of the steel coil is 3~8mm.