Roll forming fracture prevention control method for complex cross-section ultra-high strength steel thin-walled member

Abstract

The application provides a roll bending forming fracture prevention control method for a complex cross-section ultrahigh-strength steel thin-walled component and relates to the technical field of roll bending forming. The roll bending forming fracture prevention control method specifically comprises the following steps: in the whole roll bending forming process, taking the Oyane ductile fracture criterion as the evaluation standard for fracture generation, comprehensively considering the fracture prevention control by combining the roll bending forming strategy and the roll bending forming process parameters, when the process parameters of the machine set are configured, according to the difference of different machine set parameter ranges, solving the fracture prevention mathematical model, in the adjustable interval, using the process parameter adjustment method, according to the actual production conditions, setting the process parameter value range and the optimal trend value, so as to solve the optimal process parameter combination. The application can effectively prevent the roll bending forming corner fracture defects of the complex cross-section ultrahigh-strength steel thin-walled component, saves the production cost, improves the quality and the yield of the complex cross-section ultrahigh-strength steel thin-walled component, and is beneficial to the large-scale industrial production and popularization.

Description

Technical Field

[0001] This invention relates to the technical field of roll forming, and in particular to a method for preventing and controlling fractures in roll forming of ultra-high strength steel thin-walled components with complex cross-sections. Background Technology

[0002] In today's world, the automotive industry is booming. As one of the important pillar industries of national economic development, the automotive industry is still severely hampered by the global energy and environmental crisis. Lightweighting of automobiles has become an inevitable path for the sustainable development of the automotive industry.

[0003] The low cost and high performance of ultra-high strength steel have led to its increasingly widespread application in the production of lightweight automotive products. The roll forming process for ultra-high strength steel offers advantages such as high production efficiency, low manufacturing cost, and the ability to produce products with uniform cross-sectional shape, high dimensional accuracy, diverse varieties, and complex cross-sections. This process maximizes product strength and has become one of the important forming technologies for processing metal products with complex cross-sections and thin walls.

[0004] However, in the process of improving the lightweight level of roll-bent profiles by using ultra-high strength steel, a series of technical problems have arisen in the processing and manufacturing process due to the characteristics of ultra-high strength steel, such as high yield strength ratio, low elongation, poor plasticity, and more complex mechanical properties at room temperature, as well as the increasingly complex and precise technical requirements of product cross-section design. Among them, the roll bending failure of ultra-high strength steel complex cross-section thin-walled components is one of the most prominent problems.

[0005] The occurrence of fracture during roll forming reduces the yield of products, increases production costs, and jeopardizes corporate profits, seriously affecting the application of ultra-high strength steel roll forming in the field of automotive lightweighting.

[0006] During roll forming, corner cracks can occur when producing rectangular tubes from round to square shapes using ordinary carbon steel. The traditional approach is to modify the process by using a "square to square" method, employing a slitting strip to complete the roll forming process according to the product's cross-sectional shape, followed by welding and sizing. This significantly reduces stress concentration at the corners, promotes metal flow in the rounded corners, and prevents corner breakage. Building on this, a composite process combining "square to square" and "round to square" has emerged to address corner breakage in simple irregularly shaped tubes. This replaces either a single "round to square" or "square to square" process. In the sizing stage, the "square to square" process is used to sizing the straight section of the top, the two side edges, and the two small bends at the top to the final size, while the "round to square" process rolls the large arc bends to the final size. This effectively solves the corner breakage problem in trapezoidal cross-section irregularly shaped tubes.

[0007] However, the above solutions are designed for simple carbon steel rectangular tubes or irregularly shaped tubes with simple cross-sectional shapes, and are not applicable to solving the fracture problem of non-tubular high-strength steel products with complex cross-sectional shapes.

[0008] Alternatively, a localized heating forming method using roll bending can be employed to address cracking issues at small bending radii or sharp corners. This method involves using a heating device to locally heat the bent areas of the strip at high frequency, transforming the blank at the corner into an austenitic structure. The heating temperature depends on the austenitic transformation temperature of various steels. This softens the deformed and critical areas, improving their plasticity and flowability, reducing the yield strength and internal stress of the strip, and effectively decreasing the forming work and the likelihood of cracks at the bends. However, this method is relatively complex to operate, has high production and maintenance costs, and is difficult to control the relationship between heating temperature and material properties. Therefore, its application is greatly limited and it has not been widely adopted. It is not suitable for cracking problems that occur in conventional unit production. For example, the paper "Analysis of Local Heating Roll Bending Forming of High Strength Steel" (Yang Wenzhi, Yan Yu, Cao Kunyang, Wang Zhi, Journal of Beijing University of Technology, Vol. 25, No. 3, September 2013) improved the local heating roll bending forming process. It requires the establishment of a finite element model based on ABAQUS finite element software and the fully coupled thermal stress analysis method for analysis and experimental research. Obviously, the relationship between heating temperature and material is relatively complex and difficult to operate. It is only applicable to U-shaped parts. It is difficult to predict whether it is applicable to other non-tubular high-strength steels with complex cross-sectional shapes.

[0009] In addressing the issue of corner fracture in thick-walled high-strength steel formed by roll bending, a forming process exists that employs a fixed roll curvature with varying bending point positions. This process determines the corner bending curvature of the roll based on the minimum wall thickness specified in the unit's product outline. This curvature is used on the upper roll for all actual bending passes. The bending angle of the raw material flat plate is determined based on the inner arc of the finished product for different wall thicknesses. Unbent portions of the raw material at each corner are left on the bottom flat plate. During the roll bending process, the bending points gradually move towards the bottom flat plate, ultimately completing the entire bending process. Due to the continuous change in the bending force point, the forming is naturally smooth, effectively solving the problem of micro-cracks at the corners of thick-walled high-strength products.

[0010] Regarding the solution to the corner fracture of thin-walled high-strength steel rectangular tubes, one method involves increasing the arc of the forming mold, increasing the actual bending angle and the number of forming cycles, and using a method of air cooling followed by water cooling. This effectively overcomes the rebound of high-strength steel, ensures the size of the outer arc and the shaping allowance for precision forming, and better guarantees the requirement of no cracks in the outer arc. However, this method requires a bending angle of less than 90°, has a relatively simple cross-sectional shape, is costly, and has low efficiency, making it unsuitable for large-scale industrial production. Summary of the Invention

[0011] The technical problem to be solved by this invention is a solution to the fracture problem in the roll bending process of high-strength steel. In principle, the solution is to increase the actual bending angle and change the mold shape to slow down the bending deformation process. However, this solution is only suitable for simple bending forming with a bending angle of less than 90°. It is difficult to prevent fracture in the bending forming of complex cross-sections of ultra-high-strength steel with higher strength. On the other hand, local heating will increase production costs and operational difficulty, consume more cooling water, and is not suitable for thin-walled high-strength steel with complex cross-sections.

[0012] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0013] A method for preventing and controlling fracture during roll bending of ultra-high strength steel thin-walled components with complex cross-sections is disclosed. The method includes: during the complete roll bending process, using the Oyane toughness fracture criterion as the evaluation standard for fracture occurrence; integrating roll bending strategies and roll bending process parameters to comprehensively consider fracture prevention and control; when configuring unit process parameters, based on the differences in parameter ranges between different units, solving the problem through a fracture prevention mathematical model; employing process parameter adjustment methods within the adjustable range; and setting the range and optimal trend value of process parameters according to actual on-site production conditions, thereby solving for the optimal combination of process parameters.

[0014] Preferably, the thickness of the complex cross-section ultra-high strength steel thin-walled component is 1-2 mm.

[0015] Preferably, the comprehensive consideration of fracture prevention and control is a forming strategy design method that consists of a fixed arc long bending angle forming method, an even distribution of five boundary condition forming angles, and an increase in the number of forming passes to effectively prevent fracture during the complete roll forming process.

[0016] Preferably, the roll bending forming strategy selects a fixed arc length bending forming method among the bending forming methods;

[0017] In the angle allocation method, select the evenly distributed five boundary condition forming angle allocation method, as shown in the following formula:

[0018]

[0019] In the formula: N represents the number of forming passes, θ i Let θ be the bending angle of the i-th pass, and θ0 be the final bending angle. r It is a complementary angle;

[0020] When allocating the number of passes, appropriately increase the number of effective forming passes;

[0021] All three can effectively reduce the peak values ​​of equivalent plastic strain and stress triaxiality at critical fracture corners, ensuring that the damage value of the final pass is less than the fracture threshold.

[0022] Preferably, the ultra-high strength steel material is martensitic steel with a tensile strength exceeding 980 MPa. Considering the triaxial stress and plastic instability, the Swift-Voce hybrid hardening model is selected to characterize its wide-range true stress-true strain relationship, as shown in the following formula:

[0023]

[0024] Where: ε pl ε is the plastic strain, ε0 is the yield strain, m is the material work hardening index (m > 0), C, A and c are material parameters, α is an adjustable weighting coefficient, and σ is the true stress.

[0025] Preferably, the ultra-high strength steel material used is MS1700 ultra-high strength steel, i.e., σ s Above 1500MPa, σ b Above 1700MPa.

[0026] Preferably, the Oyane ductile fracture criterion is expressed by the following formula:

[0027]

[0028] In the formula: This is the equivalent plastic strain at fracture. The equivalent plastic strain is given by A, where A is the material parameter, C is the fracture threshold, and σ is the tensile strength. m For hydrostatic pressure, The equivalent stress is Von Mises.

[0029] Preferably, the Oyane toughness fracture criterion obtains the fracture threshold of ultra-high strength steel materials through a combination of basic experiments and numerical simulations. The fracture damage value is obtained by integrating the stress triaxiality relationship at the dangerous bend with plastic strain. The threshold is compared with the damage value to determine whether fracture occurs during roll forming.

[0030] Preferably, within the following ranges: frame spacing 500-700mm, roller spacing 240-320mm, forming speed 50-150mm / s, and friction coefficient 0.05-0.25, the process parameter adjustment method is a comprehensive process parameter adjustment method consisting of increasing frame spacing, increasing roller spacing, decreasing forming speed, and decreasing friction coefficient.

[0031] Preferably, the process parameter adjustment method specifically employs the response surface methodology to investigate the influence of roll bending process parameters on fracture. It was found that increasing the frame spacing, increasing the roll spacing, decreasing the forming speed, and decreasing the friction coefficient all reduce the fracture damage value. The frame spacing, friction coefficient, forming speed, and roll spacing are set as influencing factors, and the damage value is set as the response factor. A quadratic nonlinear regression mathematical model of the damage value with respect to the four influencing factors is fitted, as shown in the following formula:

[0032]

[0033] In the formula: f(x) is the response, α0, α i and α ij x is the regression coefficient. i and x j Let be the i-th and j-th independent variables, n represent the number of parameters, and β be the error term;

[0034] Subsequently, based on the actual production conditions and economic benefits on site, the range of values ​​and the optimal trend value were set, thereby solving for the optimal combination of process parameters and effectively reducing the fracture damage value.

[0035] Preferably, the fracture corner damage value obtained by the optimal combination of process parameters in the roll forming fracture prevention and control method is reduced by more than 37.98% compared with the original fracture corner damage value, and it also helps to improve the damage value of non-fracture corners, reducing it by more than 19.63%.

[0036] Preferably, a comparison between the simulated damage value of the roll forming fracture prevention and control method and the predicted damage value obtained by the fracture prevention mathematical model reveals that the relative error between the two is within 4.79%.

[0037] The above technical solution has at least the following advantages compared with the existing technology:

[0038] The present invention proposes a method for preventing and controlling fracture during roll bending of ultra-high strength steel thin-walled components with complex cross-sections. This method effectively solves the problem of corner fracture during roll bending of ultra-high strength steel thin-walled components with complex cross-sections, and realizes the production verification and application of fracture prevention during roll bending of ultra-high strength steel thin-walled components with complex cross-sections. It provides theoretical and practical basis for fracture prevention and control and efficient and precise forming of ultra-high strength steel thin-walled components with complex cross-sections during roll bending.

[0039] This invention comprehensively considers fracture prevention and control by adopting a forming strategy design method consisting of a fixed arc long bending angle forming method, an even distribution of five boundary conditions forming angles, and increasing the number of forming passes to effectively prevent fracture during the complete roll bending forming process.

[0040] In order to configure the process parameters of the unit, this invention solves the problem by using a fracture prevention mathematical model based on the differences in the parameter ranges of different units. Within its adjustable range, the process parameters are adjusted by increasing the spacing between the frames, increasing the spacing between the rollers, reducing the forming speed, and reducing the coefficient of friction.

[0041] The present invention reduces the fracture corner damage value by more than 37.98% compared with the original fracture corner damage value by adopting the roll bending fracture prevention and control method. It also helps to improve the damage value of non-fracture corners, reducing it by more than 19.63%. The yield of roll bending of ultra-high strength steel thin-walled components with complex cross-sections is close to 100%.

[0042] In summary, this invention can effectively prevent corner fracture defects in roll bending of ultra-high strength steel thin-walled components with complex cross-sections, save production costs, improve the quality and yield of ultra-high strength steel thin-walled components with complex cross-sections, and facilitate large-scale industrial production and promotion. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a diagram showing the cross-sectional bending angle numbering of the ultra-high strength steel thin-walled component with a complex cross-section involved in the roll bending forming fracture prevention and control method of a complex cross-section ultra-high strength steel thin-walled component in Embodiment 1 of the present invention.

[0045] Figure 2 This is a simulation diagram of the original damage value of the ultra-high strength steel thin-walled component with complex cross-section involved in the roll bending forming fracture prevention and control method of the ultra-high strength steel thin-walled component with complex cross-section in Embodiment 1 of the present invention.

[0046] Figure 3 This is a roll forming process diagram of a fracture prevention and control method used in a roll bending forming fracture prevention and control method for a complex cross-section ultra-high strength steel thin-walled component according to Embodiment 1 of the present invention.

[0047] Figure 4 This is a fracture damage value optimization design diagram under actual production conditions for the fracture prevention mathematical model in the roll bending forming fracture prevention and control method of a complex cross-section ultra-high strength steel thin-walled component of the present invention, which is an embodiment 1 of the present invention.

[0048] Figure 5 This is an optimization diagram of the unit process parameters in the roll bending forming fracture prevention and control method for a complex cross-section ultra-high strength steel thin-walled component according to Embodiment 1 of the present invention.

[0049] Figure 6 This is a simulation diagram of the damage value of the fracture prevention and control method used in the roll bending forming fracture prevention and control method of a complex cross-section ultra-high strength steel thin-walled component in Embodiment 1 of the present invention.

[0050] Figure 7 This is a comparison diagram of the damage values ​​of each bend before and after the fracture prevention and control method used in the roll bending forming fracture prevention and control method of a complex cross-section ultra-high strength steel thin-walled component in Embodiment 1 of the present invention.

[0051] Figure 8 This is a comparison diagram of the product before and after the fracture prevention and control method used in the roll bending forming fracture prevention and control method of a complex cross-section ultra-high strength steel thin-walled component of Embodiment 1 of the present invention. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0053] Example 1

[0054] A method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections, wherein the complex cross-section ultra-high strength steel thin-walled component is a certain type of ultra-high strength steel automotive bumper thin-walled structural part with a complex cross-section, and its product cross-sectional shape and bend angle number are as follows. Figure 1 As shown; the material is MS1700 ultra-high strength steel, i.e., σ s =1545MPa,σ b =1718MPa, slab thickness is 1.45mm, overall dimensions are 176.6mm×43.54mm, fracture phenomenon exists at C corner, fracture optimization is performed at C corner.

[0055] The method for preventing and controlling fracture during roll bending specifically includes the following steps:

[0056] First, the constitutive relation of MS1700 ultra-high strength steel was obtained using the Swift-Voce hybrid hardening model. In this embodiment, martensitic steel with a tensile strength exceeding 980 MPa was used. Considering the triaxial stress and plastic instability, the formula for characterizing its large-scale true stress-true strain relationship using the Swift-Voce hybrid hardening model is as follows:

[0057]

[0058] Where: ε pl ε is the plastic strain, ε0 is the yield strain, m is the material work hardening index (m > 0), C, A and c are material parameters, α is an adjustable weighting coefficient, and σ is the true stress.

[0059] Then, based on the fundamental tensile tests and corresponding numerical simulations of standard uniaxial tensile specimens, plane strain specimens, large radius tensile specimens, and shear specimens, the parameters of the Oyane toughness fracture criterion are solved. The Oyane toughness fracture criterion is defined by the following formula:

[0060]

[0061] In the formula: This is the equivalent plastic strain at fracture. The equivalent plastic strain is given by A, where A is the material parameter, C is the fracture threshold, and σ is the tensile strength. m For hydrostatic pressure, Von Mises equivalent stress;

[0062] The fracture threshold of the complex cross-section ultra-high strength steel thin-walled component in this embodiment was calculated to be 0.7771.

[0063] Secondly, the fracture prevention and control should be comprehensively considered by integrating the roll bending forming strategy and roll bending forming process parameters.

[0064] The comprehensive consideration of fracture prevention and control is a forming strategy design method that employs a fixed-arc long-angle forming method, an evenly distributed five-boundary-condition forming angle allocation method, and increases the number of forming passes to effectively prevent fracture during the complete roll forming process; wherein:

[0065] The aforementioned roll bending forming strategy, such as Figure 2 As shown, based on the original damage simulation values, it can be determined that the C-angle damage value exceeds the fracture threshold. The influence of forming strategies on C-angle fracture is investigated. Based on the equivalent plastic strain, peak stress triaxiality, and fracture damage value, the variation parameters of various forming strategies are compared, and it is determined that the fixed radius, fixed arc long angle forming method should be changed to the fixed arc long angle forming method.

[0066] In the angle allocation method, select the evenly distributed five boundary condition forming angle allocation method, as shown in the following formula:

[0067]

[0068] In the formula: N represents the number of forming passes, θ i Let θ be the bending angle of the i-th pass, and θ0 be the final bending angle. r It is a complementary angle;

[0069] The angle allocation method was changed, and the original C-angle allocation of 0°-19°-38°-57°-76°-82°-88°-94°-100° was changed to 0°-8.8°-20.2°-33.1°-46.6°-60.5°-73.9°-86.1°-95.4°-100° using the evenly divided five boundary condition forming angle allocation method. The roll forming process diagram was improved using fracture prevention and control methods, such as... Figure 3 As shown;

[0070] When allocating the number of passes, appropriately increase the number of effective forming passes; for example... Figure 2 As shown, based on the original damage simulation values, it can be determined that the C-corner damage value exceeds the fracture threshold. The influence of forming strategies on C-corner fracture is investigated. Based on equivalent plastic strain, peak stress triaxiality, and fracture damage value, the changing parameters of various forming strategies are compared. It is determined that the fixed-radius, fixed-arc long-angle forming method should be changed to a fixed-arc long-angle forming method; the number of forming passes to effectively prevent fracture is increased from the original 9 passes to 10 passes.

[0071] All three of these methods can effectively reduce the peak values ​​of equivalent plastic strain and stress triaxiality at critical fracture bends, resulting in damage values ​​in the final pass being less than the fracture threshold.

[0072] Furthermore, when configuring the process parameters of the unit, based on the differences in the parameter ranges of different units, the process parameter adjustment method is adopted within its adjustable range by solving the mathematical model of fracture prevention.

[0073] Specifically, Design-Expert software was used for response surface methodology analysis. The process parameter adjustment method employed response surface methodology to investigate the influence of roll forming process parameters on fracture. It was found that increasing the frame spacing, increasing the roll spacing, decreasing the forming speed, and decreasing the friction coefficient all reduced the fracture damage value. Frame spacing, friction coefficient, forming speed, and roll spacing were set as influencing factors, and damage value as the response factor. A quadratic nonlinear regression mathematical model of the damage value with respect to the four influencing factors was fitted, as shown in the following formula:

[0074]

[0075] In the formula: f(x) is the response, α0, α i and α ij x is the regression coefficient. i and x j Let be the i-th and j-th independent variables, n represent the number of parameters, and β be the error term;

[0076] The quadratic model was selected to solve the mathematical model of fracture damage value under multiple working conditions, and the mathematical model of fracture prevention related to the C-angle fracture bending angle damage value and process parameters was obtained, as shown in formula (1).

[0077]

[0078] In the formula: A is the frame spacing, B is the friction coefficient, C is the forming speed, and D is the roller spacing.

[0079] Finally, based on the actual production conditions on site, the range of process parameters and the optimal trend value are set, thereby solving for the optimal combination of process parameters;

[0080] like Figure 4 As shown, based on actual production conditions, a mathematical model was used to consider the weight ratio of influencing factors, namely: frame spacing > roller spacing > forming speed > friction coefficient. The trend values ​​of the four influencing factors were also considered: frame spacing tends to be smaller, friction coefficient tends to be larger, forming speed tends to be larger, and roller spacing tends to be smaller. This led to the optimization design of the C-angle fracture damage value under actual production conditions. The minimum fracture damage value within the upper and lower limits was solved, and a combination of process parameters that met the actual production conditions and were easy to implement was determined. These parameters were: frame spacing 607mm, friction coefficient 0.207, forming speed 129mm / s, and roller spacing 282mm. The unit parameters were then reconfigured and optimized. After finite element simulation, the C-angle damage value decreased from 0.919 before the improvement to 0.570 after the improvement, which is less than the fracture threshold of 0.7771. The optimization effect exceeded 37.98%, demonstrating a significant preventative effect.

[0081] The increase in frame spacing was 4.66%, the increase in roller spacing was 8.46%, the increase in forming speed was 29%, and the increase in friction coefficient was 38%. As the main factors influencing damage value, the increase in frame spacing and roller spacing significantly promoted a reduction in damage value, thus offsetting the negative effect of increased damage value caused by the increase in friction coefficient (a secondary factor) in forming speed. Furthermore, the increase in forming speed and friction coefficient is more conducive to controlling production costs and improving production efficiency. Therefore, this method meets the requirements of reducing damage value and preventing breakage while also improving product production efficiency.

[0082] The method in this embodiment uses the magnitude of the damage value to characterize the fracture phenomenon. The original products all had C-corner fracture problems with a pass rate of 0%. After optimization by the roll bending fracture prevention and control method for complex cross-section ultra-high strength steel thin-walled components, the C-corner fracture problem was eliminated, and the pass rate was close to 100%.

[0083] Example 2

[0084] A method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections, wherein the complex cross-section ultra-high strength steel thin-walled component is a certain type of ultra-high strength steel automotive bumper thin-walled structural part with a complex cross-section, and its product cross-sectional shape and bend angle number are as follows. Figure 1As shown; the material is MS1700 ultra-high strength steel, i.e., σ s =1536MPa,σ b =1705MPa, slab thickness is 1.45mm, overall dimensions are 176.6mm×43.54mm, fracture phenomenon exists in H corner, fracture optimization of H corner is performed.

[0085] The method for preventing and controlling fracture during roll bending specifically includes the following steps:

[0086] First, the constitutive relation of MS1700 ultra-high strength steel was obtained using the Swift-Voce hybrid hardening model. In this embodiment, martensitic steel with a tensile strength exceeding 980 MPa was used. Considering the triaxial stress and plastic instability, the formula for characterizing its large-scale true stress-true strain relationship using the Swift-Voce hybrid hardening model is as follows:

[0087]

[0088] Where: ε pl ε is the plastic strain, ε0 is the yield strain, m is the material work hardening index (m > 0), C, A and c are material parameters, α is an adjustable weighting coefficient, and σ is the true stress.

[0089] Then, based on the fundamental tensile tests and corresponding numerical simulations of standard uniaxial tensile specimens, plane strain specimens, large radius tensile specimens, and shear specimens, the parameters of the Oyane toughness fracture criterion are solved. The Oyane toughness fracture criterion is defined by the following formula:

[0090]

[0091] In the formula: This is the equivalent plastic strain at fracture. The equivalent plastic strain is given by A, where A is the material parameter, C is the fracture threshold, σm is the hydrostatic pressure, and σm is the Von Mises equivalent stress.

[0092] The fracture threshold of the complex cross-section ultra-high strength steel thin-walled component in this embodiment was calculated to be 0.7771.

[0093] Secondly, the fracture prevention and control should be comprehensively considered by integrating the roll bending forming strategy and roll bending forming process parameters.

[0094] The comprehensive consideration of fracture prevention and control is a forming strategy design method that employs a fixed-arc long-angle forming method, an evenly distributed five-boundary-condition forming angle allocation method, and increases the number of forming passes to effectively prevent fracture during the complete roll forming process; wherein:

[0095] The aforementioned roll bending forming strategy, such as Figure 2As shown, based on the original damage simulation values, it can be determined that the damage value of the H-angle exceeds the fracture threshold. The influence of forming strategies on H-angle fracture is investigated. Based on the equivalent plastic strain, peak stress triaxiality, and fracture damage value, the variation parameters of various forming strategies are compared, and it is determined that the fixed radius, fixed arc long bend forming method should be changed to the fixed arc long bend forming method.

[0096] In the angle allocation method, select the evenly distributed five boundary condition forming angle allocation method, as shown in the following formula:

[0097]

[0098] In the formula: N represents the number of forming passes, θ i Let θ be the bending angle of the i-th pass, and θ0 be the final bending angle. r It is a complementary angle;

[0099] The angle allocation method was changed, and the original H-angle allocation of 0°-11°-21°-31°-41°-51°-62°-81°-100° was changed to 0°-8.8°-20.2°-33.1°-46.6°-60.5°-73.9°-86.1°-95.4°-100° using the evenly divided five boundary condition forming angle allocation method. The roll forming process diagram was improved using fracture prevention and control methods, such as... Figure 3 As shown;

[0100] When allocating the number of passes, appropriately increase the number of effective forming passes; for example... Figure 2 As shown, based on the original damage simulation values, it can be determined that the damage value of the H-angle exceeds the fracture threshold. The influence of forming strategies on H-angle fracture is investigated. Based on the equivalent plastic strain, peak stress triaxiality, and fracture damage value, the changing parameters of various forming strategies are compared. It is determined that the fixed-radius, fixed-arc long-angle forming method should be changed to a fixed-arc long-angle forming method; the number of forming passes to effectively prevent fracture is increased from the original 9 passes to 10 passes.

[0101] All three of these methods can effectively reduce the peak values ​​of equivalent plastic strain and stress triaxiality at critical fracture bends, resulting in damage values ​​in the final pass being less than the fracture threshold.

[0102] Furthermore, when configuring the process parameters of the unit, based on the differences in the parameter ranges of different units, the process parameter adjustment method is adopted within its adjustable range by solving the mathematical model of fracture prevention.

[0103] Specifically, Design-Expert software was used for response surface methodology analysis. The process parameter adjustment method employed response surface methodology to investigate the influence of roll forming process parameters on fracture. It was found that increasing the frame spacing, increasing the roll spacing, decreasing the forming speed, and decreasing the friction coefficient all reduced the fracture damage value. Frame spacing, friction coefficient, forming speed, and roll spacing were set as influencing factors, and damage value as the response factor. A quadratic nonlinear regression mathematical model of the damage value with respect to the four influencing factors was fitted, as shown in the following formula:

[0104]

[0105] In the formula: f(x) is the response, α0, α i and α ij x is the regression coefficient. i and x j Let be the i-th and j-th independent variables, n represent the number of parameters, and β be the error term;

[0106] The quadratic model was selected to solve the mathematical model of fracture damage value under multiple working conditions, and the fracture prevention mathematical model related to the C-angle and H-angle fracture bending angle damage values ​​and process parameters was obtained, as shown in formula (2).

[0107]

[0108] In the formula: A is the frame spacing, B is the friction coefficient, C is the forming speed, and D is the roller spacing.

[0109] Finally, based on the actual production conditions on site, the range of process parameters and the optimal trend value are set, thereby solving for the optimal combination of process parameters;

[0110] like Figure 4 As shown, based on actual production conditions, a mathematical model was used to consider the weight ratio of influencing factors, namely: frame spacing > roller spacing > forming speed > friction coefficient. The trend values ​​of the four influencing factors were also considered: frame spacing tends to be smaller, friction coefficient tends to be larger, forming speed tends to be larger, and roller spacing tends to be smaller. This led to the optimization design of the H-angle fracture damage value under actual production conditions. The minimum fracture damage value within the upper and lower limits was determined, and a combination of process parameters that met the actual production conditions and facilitated effective implementation was found: frame spacing 605mm, friction coefficient 0.142, forming speed 121mm / s, and roller spacing 278mm. The unit parameters were then reconfigured and optimized. After finite element simulation, the H-angle damage value decreased from 0.945 before the improvement to 0.575 after the improvement, which is less than the fracture threshold of 0.7771. The optimization effect exceeded 39.15%, demonstrating a significant preventative effect.

[0111] The increase in frame spacing was 4.31%, the increase in roller spacing was 6.92%, the increase in forming speed was 21%, and the decrease in the coefficient of friction was 5.33%. Under the combined effect of increasing the frame and roller spacing (the main influencing factors of damage) and decreasing the coefficient of friction (a secondary influencing factor), the damage value was significantly reduced, offsetting the negative effect of increased damage value caused by increased forming speed. Furthermore, increasing the forming speed helps control production costs and improve production efficiency. Therefore, this method meets the requirements of reducing damage value and preventing breakage while also improving product production efficiency.

[0112] The method in this embodiment uses the magnitude of the damage value to characterize the fracture phenomenon. The initial products all had H-angle fracture problems and the yield was 0%. After optimization by the method of preventing and controlling fracture of ultra-high strength steel thin-walled components with complex cross-sections, the H-angle fracture problem was eliminated and the yield was close to 100%.

[0113] Example 3

[0114] A method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections, wherein the complex cross-section ultra-high strength steel thin-walled component is a certain type of ultra-high strength steel automotive bumper thin-walled structural part with a complex cross-section, and its product cross-sectional shape and bend angle number are as follows. Figure 1 As shown; the material is MS1700 ultra-high strength steel, i.e., σ s =1533MPa,σ b =1703MPa, slab thickness is 1.45mm, overall dimensions are 176.6mm×43.54mm, fracture phenomenon exists at C corner and H corner, fracture optimization is performed on C corner and H corner simultaneously.

[0115] The method for preventing and controlling fracture during roll bending specifically includes the following steps:

[0116] First, the constitutive relation of MS1700 ultra-high strength steel was obtained using the Swift-Voce hybrid hardening model. In this embodiment, martensitic steel with a tensile strength exceeding 980 MPa was used. Considering the triaxial stress and plastic instability, the formula for characterizing its large-scale true stress-true strain relationship using the Swift-Voce hybrid hardening model is as follows:

[0117]

[0118] Where: ε pl ε is the plastic strain, ε0 is the yield strain, m is the material work hardening index (m > 0), C, A and c are material parameters, α is an adjustable weighting coefficient, and σ is the true stress.

[0119] Then, based on the fundamental tensile tests and corresponding numerical simulations of standard uniaxial tensile specimens, plane strain specimens, large radius tensile specimens, and shear specimens, the parameters of the Oyane toughness fracture criterion are solved. The Oyane toughness fracture criterion is defined by the following formula:

[0120]

[0121] In the formula: This is the equivalent plastic strain at fracture. The equivalent plastic strain is given by A, where A is the material parameter, C is the fracture threshold, and σ is the tensile strength. m For hydrostatic pressure, Von Mises equivalent stress;

[0122] The fracture threshold of the complex cross-section ultra-high strength steel thin-walled component in this embodiment was calculated to be 0.7771.

[0123] Secondly, the fracture prevention and control should be comprehensively considered by integrating the roll bending forming strategy and roll bending forming process parameters.

[0124] The comprehensive consideration of fracture prevention and control is a forming strategy design method that employs a fixed-arc long-angle forming method, an evenly distributed five-boundary-condition forming angle allocation method, and increases the number of forming passes to effectively prevent fracture during the complete roll forming process; wherein:

[0125] The aforementioned roll bending forming strategy, such as Figure 2 As shown, based on the original damage simulation values, it can be determined that the damage values ​​at the C-angle and H-angle exceed the fracture threshold. The influence of forming strategies on the fracture at the C-angle and H-angle is investigated. Based on the equivalent plastic strain, peak stress triaxiality, and fracture damage values, the variation parameters of various forming strategies are compared, and it is determined that the fixed-radius, fixed-arc long-angle forming method should be changed to the fixed-arc long-angle forming method.

[0126] In the angle allocation method, select the evenly distributed five boundary condition forming angle allocation method, as shown in the following formula:

[0127]

[0128] In the formula: N represents the number of forming passes, θ i Let θ be the bending angle of the i-th pass, and θ0 be the final bending angle. r It is a complementary angle;

[0129] The angle allocation method was changed. Using the evenly divided five-boundary-condition forming angle allocation method, the original C-angle allocation of 0°-19°-38°-57°-76°-82°-88°-94°-100° was changed to 0°-8.8°-20.2°-33.1°-46.6°-60.5°-73.9°-86.1°-95.4°-100°. The original H-angle allocation of 0°-11°-21°-31°-41°-51°-62°-81°-100° was changed to 0°-8.8°-20.2°-33.1°-46.6°-60.5°-73.9°-86.1°-95.4°-100°. A fracture prevention and control method was adopted to improve the roller knurling process diagram, such as... Figure 3 As shown;

[0130] When allocating the number of passes, appropriately increase the number of effective forming passes; for example... Figure 2 As shown, based on the original damage simulation values, it can be determined that the damage values ​​at the C-angle and H-angle exceed the fracture threshold. The influence of forming strategies on the fracture at the C-angle and H-angle is investigated. Based on the equivalent plastic strain, peak stress triaxiality, and fracture damage values, the changing parameters of various forming strategies are compared. It is determined that the fixed-radius, fixed-arc long-angle forming method should be changed to a fixed-arc long-angle forming method; the number of forming passes to effectively prevent fracture is increased from the original 9 passes to 10 passes.

[0131] All three of these methods can effectively reduce the peak values ​​of equivalent plastic strain and stress triaxiality at critical fracture bends, resulting in damage values ​​in the final pass being less than the fracture threshold.

[0132] Furthermore, when configuring the process parameters of the unit, based on the differences in the parameter ranges of different units, the process parameter adjustment method is adopted within its adjustable range by solving the mathematical model of fracture prevention.

[0133] Specifically, Design-Expert software was used for response surface methodology analysis. The process parameter adjustment method employed response surface methodology to investigate the influence of roll forming process parameters on fracture. It was found that increasing the frame spacing, increasing the roll spacing, decreasing the forming speed, and decreasing the friction coefficient all reduced the fracture damage value. Frame spacing, friction coefficient, forming speed, and roll spacing were set as influencing factors, and damage value as the response factor. A quadratic nonlinear regression mathematical model of the damage value with respect to the four influencing factors was fitted, as shown in the following formula:

[0134]

[0135] In the formula: f(x) is the response, α0, α i and α ij x is the regression coefficient. i and x jLet be the i-th and j-th independent variables, n represent the number of parameters, and β be the error term;

[0136] The quadratic model was selected to solve the mathematical model of fracture damage value under multiple working conditions, and the fracture prevention mathematical model related to the C-angle and H-angle fracture bending angle damage values ​​and process parameters was obtained, as shown in formula (1) and formula (2).

[0137]

[0138]

[0139] In the formula: A is the frame spacing, B is the friction coefficient, C is the forming speed, and D is the roller spacing.

[0140] Finally, based on the actual production conditions on site, the range of process parameters and the optimal trend value are set, thereby solving for the optimal combination of process parameters;

[0141] like Figure 4 As shown, based on actual production conditions, a mathematical model is used to consider the weight ratio of influencing factors, namely: frame spacing > roller spacing > forming speed > friction coefficient. The trend values ​​of the four influencing factors are also considered: frame spacing tends to be smaller, friction coefficient tends to be larger, forming speed tends to be larger, and roller spacing tends to be smaller. Therefore, a joint optimization design for the C-angle and H-angle fracture damage values ​​under actual production conditions is performed to solve for the minimum fracture damage value within the upper and lower limits, and to obtain a combination of process parameters that meets the actual production conditions and is easy to implement effectively. This combination is: frame spacing 627mm, friction coefficient 0.129, forming speed 109mm / s, roller spacing 288mm. The unit parameters are then re-optimized, as follows: Figure 5 As shown.

[0142] like Figure 6 As shown, the damage value simulation diagram obtained using the fracture prevention and control method shows that the overall damage value distribution still shows a trend of increasing damage value at the bend, which is consistent with the change law of roll bending, but the value is significantly reduced.

[0143] like Figure 7 As shown, the damage values ​​of each bend after the improvement of the prevention method are compared with the original damage values. After adopting the optimal fracture prevention method, the damage values ​​of all nine bends of the ultra-high strength steel complex cross-section thin-walled component are reduced compared with the original simulation results. In particular, the C-angle and H-angle, where fracture occurs, show a greater reduction in damage values ​​due to the application of targeted roll bending forming strategy and fracture prevention method of forming process parameters. The damage values ​​are reduced from 0.919 and 0.945 before the improvement to 0.488 and 0.509 after the improvement, respectively, with optimization effects of 46.90% and 46.14%, respectively, showing a significant prevention effect.

[0144] Meanwhile, comparing the simulation results of damage values ​​at C-angle and H-angle with the predicted values ​​obtained from the fracture prevention mathematical model, it can be found that the relative errors of the two are both within 5%, which verifies the rationality of the fracture prevention mathematical model and the effectiveness of the fracture prevention and control method.

[0145] like Figure 8 As shown, the industrial application of the fracture prevention and control method for roll bending of ultra-high strength steel thin-walled components with complex cross-sections was completed and a product was obtained. The focus was on the C-corner and H-corner, which have fracture defects. The result is that the corner quality is good and no fracture phenomenon occurs. This shows that the fracture prevention and control method based on the optimization of forming strategy and process parameters provided in this embodiment has effectively achieved the predetermined goal of eliminating corner fracture during roll bending of ultra-high strength steel thin-walled components with complex cross-sections.

[0146] After optimizing both C-angle and H-angle, the increase in frame spacing was 8.10%, the increase in roller spacing was 10.77%, the increase in forming speed was 23%, and the decrease in friction coefficient was 14%. Under the combined effect of increasing frame spacing and roller spacing (the main influencing factors of damage value) and decreasing friction coefficient (a secondary influencing factor), the damage value was significantly reduced, offsetting the negative effect of increased damage value caused by increased forming speed. On the other hand, increasing forming speed is more conducive to controlling production costs and improving production efficiency. Therefore, this method meets the requirements of reducing damage value and preventing breakage while also improving product production efficiency.

[0147] The method in this embodiment uses the magnitude of the damage value to characterize the fracture phenomenon. The initial products all have C-angle and H-angle fracture problems with a yield of 0%. After optimization by the method of preventing and controlling fracture of ultra-high strength steel thin-walled components with complex cross-sections, there are no fracture problems at the C-angle and H-angle, and the yield is close to 100%.

[0148] The present invention proposes a method for preventing and controlling fracture during roll bending of ultra-high strength steel thin-walled components with complex cross-sections. This method effectively solves the problem of corner fracture during roll bending of ultra-high strength steel thin-walled components with complex cross-sections, and realizes the production verification and application of fracture prevention during roll bending of ultra-high strength steel thin-walled components with complex cross-sections. It provides theoretical and practical basis for fracture prevention and control and efficient and precise forming of ultra-high strength steel thin-walled components with complex cross-sections during roll bending.

[0149] This invention comprehensively considers fracture prevention and control by adopting a forming strategy design method consisting of a fixed arc long bending angle forming method, an even distribution of five boundary conditions forming angles, and increasing the number of forming passes to effectively prevent fracture during the complete roll bending forming process.

[0150] In order to configure the process parameters of the unit, this invention solves the problem by using a fracture prevention mathematical model based on the differences in the parameter ranges of different units. Within its adjustable range, the process parameters are adjusted by increasing the spacing between the frames, increasing the spacing between the rollers, reducing the forming speed, and reducing the coefficient of friction.

[0151] The present invention reduces the fracture corner damage value by more than 37.98% compared with the original fracture corner damage value by adopting the roll bending fracture prevention and control method. It also helps to improve the damage value of non-fracture corners, reducing it by more than 19.63%. The yield of roll bending of ultra-high strength steel thin-walled components with complex cross-sections is close to 100%.

[0152] In summary, this invention can effectively prevent corner fracture defects in roll bending of ultra-high strength steel thin-walled components with complex cross-sections, save production costs, improve the quality and yield of ultra-high strength steel thin-walled components with complex cross-sections, and facilitate large-scale industrial production and promotion.

[0153] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections, characterized in that, The ultra-high strength steel material mentioned above is martensitic steel with a tensile strength exceeding 980 MPa; The method for preventing and controlling fracture during roll forming includes: using the Oyane toughness fracture criterion as the evaluation standard for fracture occurrence during the complete roll forming process; integrating roll forming strategies and roll forming process parameters to comprehensively consider fracture prevention and control; when configuring the process parameters, based on the differences in the range of process parameters for different units, solving the mathematical model for fracture prevention; using the process parameter adjustment method within its adjustable range; and setting the range of process parameter values ​​and the optimal trend value based on the actual production conditions on site, thereby solving for the optimal combination of process parameters. The comprehensive consideration of fracture prevention and control is a forming strategy design method consisting of a fixed arc long bending angle forming method, an even distribution of five boundary conditions forming angle, and an increase in the number of forming passes to effectively prevent fracture during the complete roll forming process. The aforementioned roll bending forming strategy selects a fixed arc length bend forming method among the bend forming methods; In the angle allocation method, select the evenly distributed five boundary condition forming angle allocation method, as shown in the following formula: ； In the formula: To form a course, For the first Curve angle of each track, The final bending angle, It is a complementary angle; When allocating the number of passes, appropriately increase the number of effective forming passes; The fixed-arc long-angle forming method, the evenly distributed five-boundary-condition forming angle distribution method, and the addition of the complete roll bending forming process can all effectively reduce the equivalent plastic strain and stress triaxiality peak value at the critical node of the fractured bend, so that the damage value of the final pass is less than the fracture threshold. The aforementioned process parameter adjustment method specifically employs the response surface methodology to investigate the influence of roll bending process parameters on fracture. Increasing the frame spacing, increasing the roller spacing, decreasing the forming speed, and decreasing the friction coefficient are used to reduce fracture damage. Frame spacing, friction coefficient, forming speed, and roller spacing are set as influencing factors, and damage value is set as the response. A quadratic nonlinear regression mathematical model of the damage value with respect to the four influencing factors is fitted, as shown in the following formula: ； In the formula: In response, , and For regression coefficients, and For the first The and the first One influencing factor, Indicates the number of parameters. This is the error term; Based on the actual production conditions on site, a mathematical model is used to consider the weight ratio of influencing factors, namely: frame spacing > roller spacing > forming speed > friction coefficient, as well as the trend values ​​of the four influencing factors, namely, the frame spacing tends to be smaller, the friction coefficient tends to be larger, the forming speed tends to be larger, and the roller spacing tends to be smaller.

2. The method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections according to claim 1, characterized in that, The method further includes: selecting the Swift-Voce hybrid hardening model to characterize the large-scale true stress-true strain relationship of ultra-high strength steel, as shown in the following formula: ； In the formula: For plastic strain, For yield strain, The work hardening index of the material. , , and For material parameters, These are adjustable weighting coefficients. This is the true stress.

3. The method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections according to claim 1, characterized in that, The Oyane ductile fracture criterion is given by the following formula: ； In the formula: This is the equivalent plastic strain at fracture. The equivalent plastic strain is given by A, which is a material parameter. The fracture threshold, For hydrostatic pressure, The equivalent stress is Von Mises.

4. The method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections according to claim 1, characterized in that, The Oyane toughness fracture criterion obtained the fracture threshold of ultra-high strength steel by combining basic experiments and numerical simulations. The fracture damage value is obtained by integrating the stress triaxiality relationship at the dangerous bend with plastic strain. The threshold and the damage value are compared to determine whether fracture occurs during roll forming.

5. The method for preventing and controlling roll bending fracture of ultra-high strength steel thin-walled components with complex cross-sections according to claim 1, characterized in that, The optimal combination of process parameters reduces the fracture corner damage value by more than 37.98% compared to the original fracture corner damage value, and also helps to improve the damage value of non-fracture corners by more than 19.63%.

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