Finite element simulation method for analyzing plastic strain transfer of gradient structural steel

Abstract

The invention discloses a finite element simulation method for analyzing plastic strain transfer of gradient structural steel, and belongs to the technical field of metal material analysis. According to the method, engineering stress-strain curves of different parts of the gradient structural steel are tested by taking performance change reflected by morphology and structure change of the gradient structural steel as a starting point so as to obtain material attributes, a gradient structural finite element simulation model is designed, and a reasonable transition connection mode is set at an interface where the material attributes change; the strain distribution and transmission process of the gradient structural steel is analyzed, a foundation can be provided for clarification of a plastic deformation gradient transmission mechanism of the gradient structural steel, and the mechanical property of the gradient structural steel is further improved on the premise that efficiency is improved and cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of metal material analysis, and in particular to a finite element simulation method for analyzing plastic strain transfer of gradient structural steel. Background Art

[0002] Gradient structure materials are new materials developed in recent years inspired by biological structures such as bones and shells. The gradient structure shows continuous gradient changes in structure at the microscopic level, which makes the performance of the material also show gradient changes. The strain gradient generated by gradient structure metal materials during plastic deformation will promote the accumulation of geometric necessary dislocations and inhibit strain localization, improve work hardening ability, and play a key role in improving its performance and inhibiting plastic instability. In recent years, the research on gradient structure metal materials has quickly become the focus of research by scholars from various countries, and has been increasingly widely used in the research on improving the performance of engineering materials.

[0003] For metal materials, their gradient structures can be roughly divided into two categories: structural gradients such as grain size gradient, lamellar thickness gradient and twin thickness gradient; chemical gradients such as phase gradient, chemical composition gradient and precipitate gradient. The organization with both the above two types of gradient structure characteristics is called a mixed gradient structure, such as large-size steel products. The phase transformation law related to the cooling rate gradient makes gradient structure steel products or components more common in industrial production, such as medium and thick plates, rollers, shafts, gears and other large-size steel products. Clarifying the strain transfer law of gradient structure metals during plastic deformation is the key to clarifying the mechanism of gradient structure improving the plastic deformation ability and work hardening ability of metal materials. At present, the relevant research on gradient metals is mainly focused on structural gradient metals, and they are usually single-phase metal materials, such as copper alloys, titanium alloys and nickel alloys. There is little research on the plastic strain transfer law of gradient structure steel. The main means to study the plastic strain transfer of gradient structure include microscopic digital image correlation technology (Digital Image Correlation, DIC), in-situ high-energy X-ray diffraction technology (HE-XRD) based on synchrotron radiation and finite element analysis. At present, the microscopic DIC technology is still difficult to distinguish the microstructure in complex phase steels, and its actual observation results are still mostly macroscopic and mesoscopic strain gradient distributions, which cannot identify the microscopic process of plastic strain transfer in gradient structural steels. Although in-situ HE-XRD technology can analyze microscopic strain transfer, its experimental resources are difficult to obtain.

[0004] Therefore, the easy-to-carry finite element simulation has become a convenient way to analyze the plastic strain transfer of gradient structures. However, there are few finite element analysis methods for the plastic strain transfer of gradient structural steel. By testing the stress-strain curves of different parts of the gradient structural steel to obtain the material properties, the simulation model is established in combination with the finite element software, and a reasonable transition connection method is selected near the location where the material properties change. The advantages of nonlinear calculation and visualization of finite element analysis are used to analyze the strain distribution and transfer process of the gradient structural steel, which provides a basis for clarifying the gradient transfer mechanism of plastic deformation of gradient structural steel. In addition, new gradient structural steels can be designed based on the finite element calculation results, which helps to further improve the mechanical properties of gradient structural steels while improving efficiency and reducing costs. To this end, a finite element simulation method for analyzing the plastic strain transfer of gradient structural steel is proposed. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to set the interface connection and transition mode of the finite element model of gradient structural steel so that a reasonable transition can be produced at the position where the material properties change, so as to accurately analyze the distribution of strain and its transmission process, and provide a finite element simulation method for analyzing the plastic strain transmission of gradient structural steel.

[0006] like Fig.10 As shown, the present invention solves the above technical problems through the following technical solutions, and the present invention comprises the following steps:

[0007] S1: Determination of Gradient Structural Steel Material Properties

[0008] Determine the material properties of each part of the gradient structure according to the change in mechanical properties corresponding to the change in the microstructure of the gradient structure material;

[0009] S2: Finite Element Geometry Model for Designing Graded Structural Steel

[0010] Establish a finite element geometric model according to the sizes of different gradient structure samples, assign material properties to different regions of the gradient structure samples, determine the connection mode and boundary conditions, and divide the grid;

[0011] S3: Analyzing the strain transfer information of gradient structural steel

[0012] Based on the strain cloud diagram along the tensile direction obtained by simulation post-processing, the strain transfer mode along the direction of gradient structure change during plastic deformation is analyzed, and the simulation results are verified by combining with measured data.

[0013] Furthermore, in step S1, for the gradient transition of the microstructure presented by the gradient structural steel, the mechanical properties corresponding to different parts are also different. The engineering stress-strain curves of different parts of the gradient structural steel are tested by room temperature tensile test to determine the material properties of each part of the gradient structure.

[0014] Furthermore, in step S1, in order to accurately reflect the plastic deformation characteristics of the material and improve the simulation accuracy, the engineering stress-strain curve is converted into a true stress-strain curve, and the true stress-strain curve obtained in the plastic deformation stage after the yield point is selected as the basic material property to be substituted into the simulation software abaqus.

[0015] Furthermore, in step S2, a corresponding finite element simulation model is constructed based on the actual stretching sample size, the material properties of each part obtained in step S1 are sequentially substituted into the corresponding divided areas in the finite element simulation model, and a transition layer is set at the boundary where the material properties change. The relative deformation and movement between two adjacent parts are constrained according to the actual contact area between the parts through tie connection, boundary conditions are assigned, and the grid size and unit type are determined.

[0016] Furthermore, the material property ΔX of the transition layer is:

[0017]

[0018] Among them, X1 and X2 are the basic material properties of the two adjacent interfaces of the transition layer.

[0019] 6. According to a finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 4, it is characterized in that the mesh unit type is a hexahedral unit, the mesh is locally refined in the transition layer set where the material properties change, the mesh size is 0.1 mm, and the mesh size of the remaining area is 0.5 mm.

[0020] Furthermore, in step S3, a strain cloud diagram along the stretching direction is derived after finite element simulation calculation, the continuity of the interface connection is observed, the strain transfer mode along the direction of gradient structure change is analyzed, and the simulation results are verified to calculate their accuracy.

[0021] Compared with the prior art, the present invention has the following advantages: the finite element simulation method for analyzing the plastic strain transfer of gradient structural steel takes the performance changes reflected by the changes in the morphology and organization of the gradient structural steel as the starting point, tests the engineering stress-strain curves of different parts of the gradient structural steel to obtain material properties, designs a gradient structure finite element simulation model and sets a reasonable transition connection method at the interface where the material properties change, analyzes the strain distribution and transfer process of the gradient structural steel, which can provide a basis for clarifying the plastic deformation gradient transfer mechanism of the gradient structural steel, and help to further improve the mechanical properties of the gradient structural steel while improving efficiency and reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a diagram of a variable cross-section steel plate in Embodiment 1 of the present invention;

[0023] Figure 2 Schematic diagram of the gradient structure microstructure in the embodiment of the present invention; (a) is a schematic diagram of the gradient structure microstructure along the length direction of the sample in the first embodiment, wherein (1) is the microstructure morphology near the hot-rolled end of the tensile sample, (2) is the microstructure morphology in the middle of the tensile sample, and (3) is the microstructure morphology near the warm-rolled end of the tensile sample; (b) is a schematic diagram of the gradient structure microstructure along the thickness direction of the sample in the second embodiment, wherein (1) is the fine-grained microstructure morphology at the surface sampling position of the steel block, (2) is the microstructure morphology at the transition sampling position in the middle of the steel block, and (3) is the coarse-grained microstructure morphology at the core sampling position of the steel block;

[0024] Figure 3 Dimensional diagrams of tensile specimens in the embodiments of the present invention; (a) is a dimensional diagram of the tensile specimen in Embodiment 1, and (b) is a dimensional diagram of the tensile specimen in Embodiment 2;

[0025] Figure 4 1 is an engineering stress-strain curve characterizing material parameters of various parts in the embodiments of the present invention, (a) is the engineering stress-strain curve in the first embodiment, and (b) is the engineering stress-strain curve in the second embodiment;

[0026] Figure 5 The Vickers hardness diagram along the layer thickness direction in Example 2 of the present invention; (a) is the actual microhardness of the sample with a layer thickness of 3.6 mm after cutting, and (b) is the microhardness of the whole sample before cutting;

[0027] Figure 6 Schematic diagrams of finite element geometric models in embodiments of the present invention, (a) is a schematic diagram of the finite element geometric model in Embodiment 1, (b) is a schematic diagram of the finite element geometric model in Embodiment 2;

[0028] Figure 7The finite element strain nephograms along the tensile direction are derived in the embodiments of the present invention; (a) is the strain nephogram in the first embodiment, wherein (1) is the strain nephogram when ε=1%, (2) is the strain nephogram when ε=3%, and (3) is the strain nephogram when ε=11%; (b) is the strain nephogram in the second embodiment, wherein (1) is the strain nephogram when ε=0.5%, (2) is the strain nephogram when ε=1.8%, and (3) is the strain nephogram when ε=4%;

[0029] Figure 8 The corresponding embodiment of the present invention is Figure 7 (a) Full-field strain measurement photo along the tensile direction, (a) is the strain nephogram corresponding to ε = 1%, (b) is the strain nephogram corresponding to ε = 3%, and (c) is the strain nephogram corresponding to ε = 11%;

[0030] Fig. 9 A comparison diagram of finite element derived and measured engineering stress-strain curves of gradient structural steel in the strain range of 5% in Example 2 of the present invention;

[0031] Fig.10 The present invention is a schematic flow chart of the finite element simulation method for analyzing plastic strain transfer of gradient structural steel. DETAILED DESCRIPTION

[0032] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiment.

[0033] Embodiment 1

[0034] This embodiment provides a technical solution: a finite element simulation method for analyzing plastic strain transfer of gradient structural steel, comprising the following steps:

[0035] The specific process of step S1 is as follows:

[0036] According to the Chinese invention patent number CN202110099128.8, hot-rolled medium manganese steel is cut into Figure 1 The variable cross-section steel plate shown in the figure is subjected to warm rolling to prepare a gradient non-uniform structure along the length direction. After intercritical annealing, the gradient structure microstructure morphology is as follows Figure 2 As shown in (a), there is a trend of transition from hot rolling (lath-like) to warm rolling (equiaxed + lamellar);

[0037] In order to accurately obtain the material properties exhibited by the gradient structural characteristics during the stretching process, the gauge length section position of the tensile specimen is selected through the full-field strain measurement experiment, and efforts are made to capture the engineering stress and strain data of the entire stretching process to the maximum extent in this process; given that the change trend of the gradient structure is along the stretching direction of the specimen, the extraction of engineering stress and strain data must strictly follow the direction of this gradient change. The size of the tensile specimen is as follows: Figure 3 As shown in (a), the stress condition remains unchanged under uniaxial tension, and 19 sets of engineering stress-strain curves are extracted at intervals of 1 mm in the gauge length section. Figure 4 As shown in (a), material 1 represents the material properties of the gradient structural steel near the hot rolling end, which is characterized by lower yield strength and higher elongation. Material 19 represents the material properties of the gradient structural steel near the warm rolling end, which is characterized by higher yield strength and lower elongation. Material 2-material 18 are the material properties of the transition from the hot rolling end to the warm rolling end. The engineering stress-strain curves in this part are intertwined. In order to better illustrate the changes in material properties, the yield strength of the extracted material true stress-strain curve will be used to roughly understand the change trend of material properties. Considering the actual geometric changes of the material during the deformation process, in order to accurately describe the mechanical response of the material, the engineering stress-strain is converted into the true stress-strain as the basic material property for substitution into the simulation software abaqus, where:

[0038] ε t =ln(1+ε e )

[0039] σ t =σ e (1+ε e )

[0040] in:

[0041] ε t As the converted true strain, the change of the cross-sectional area of ​​the object during the deformation process is taken into account, so it can more accurately reflect the actual deformation of the object;

[0042] σ t As the converted true stress, the actual stress after the change of the cross-sectional area of ​​the material is taken into account, the difference between the true stress and the engineering stress becomes more obvious as the degree of deformation increases;

[0043] Considering that the material maintains continuity during the deformation process, in order to accurately reflect the plastic deformation characteristics of the material and improve the simulation accuracy, the true stress-strain curve obtained in the plastic deformation stage after the yield point is selected as the material parameter to be substituted into abaqus.

[0044] The obtained yield strength is shown in Table 1;

[0045] Table 1 Yield strength of various parts of gradient structural steel 1

[0046] Material Name Yield strength(MPa) Material Name Yield strength(MPa) Material 1-near hot-rolled end 794.4 Material 11 850.8 Material 2 800.8 Material 12 842.5 Material 3 811.2 Material 13 834.7 Material 4 827.3 Material 14 828.6 Material 5 838.9 Material 15 836.6 Material 6 852.4 Material 16 845.4 Material 7 855.2 Material 17 842.3 Materials 8 857.2 Material 18 834.9 Material 9 856.1 Material 19-Near warm rolling end 843.9 Materials 10 852.8

[0047] The specific process of step S2 is as follows:

[0048] A finite element geometric model was established based on the actual tensile specimen size, where the specimen gauge length L = 25 mm, width B = 5 mm, and thickness h = 1.8 mm; the model gauge section was divided into 21 spatial components according to the position corresponding to the extraction of material properties in step S1, and the material properties determined in step S1 were substituted into the model, where the elastic modulus E = 112407 MPa and the Poisson's ratio μ = 0.3; the relative deformation and movement between the two adjacent surfaces were constrained by tie connection according to the actual contact area between the components, so that the degrees of freedom of the corresponding two nodes had the same value; the model used static and general analysis steps; in the boundary condition setting, in order to maintain the consistency of direction, the hot-rolled end with lower yield strength and higher elongation was defined as a rigid constraint, and the warm-rolled end with higher yield strength and lower elongation was rigidly displaced along the tensile direction; a transition layer was set at the position where the material properties changed, and the material property ΔX of the transition layer was (where X1 and X2 were the basic material properties of the two adjacent interfaces of the transition layer, respectively):

[0049]

[0050] The unit type in the meshing process is hexahedral unit, and the mesh size is 0.5 mm. The mesh is locally refined in the transition layer set where the material properties change, and the mesh size is 0.1 mm. Figure 6 As shown in (a), the unit grid type is C3D8R;

[0051] The specific process of step S3 is as follows:

[0052] The simulation software abaqus was used to simulate the tensile process of the sample. Since the constitutive law of the warm rolling end after the true strain reached 0.16 is unknown, Figure 4 The simulation results outside the warm rolling end (a) may be unreliable; the strain contour along the tensile direction of the specimen is obtained as shown in Figure 7 As shown in (a), the strain transfer state is that the strain transfer starts from the hot-rolled end with relatively low yield strength to the warm-rolled end with relatively high yield strength when the strain is 1%-3%, and gradually transfers from the warm-rolled end to the hot-rolled end when the strain is 3%-11%. Figure 8 Compared with the actual strain distribution of full-field strain measurement, the modeling results reflected by this finite element modeling have good accuracy with the actual results and can present the strain transmission mode in the real tensile deformation process.

[0053] Embodiment 2

[0054] This embodiment provides a technical solution: a finite element simulation method for analyzing plastic strain transfer of gradient structural steel, comprising the following steps:

[0055] The specific process of step S1 is as follows:

[0056] Due to the different quenching cooling rates, the surface layer and the center layer of the steel part present two different organizational structures (see Figure 2 (b) in the figure, where the fine-grained structure of the surface has a higher yield strength and a lower elongation, and the coarse-grained structure of the core has a relatively lower yield strength and a relatively higher elongation; the engineering stress-strain curve is obtained by cutting the tensile specimens of the gradient structural steel along the thickness direction for the surface and the core (see Figure 4 (b)), where material 1 represents the material properties of the surface layer of the gradient structural steel, which is characterized by high yield strength and low elongation, material 7 represents the material properties of the core of the gradient structural steel, which is characterized by low yield strength and high elongation, and materials 2-6 are the material properties of the middle transition, and the closer to the surface, the higher the yield strength and the lower the elongation; further, the engineering stress and strain are converted into true stress and strain as the basic material properties for the simulation software abaqus, where:

[0057] ε t =ln(1+ε e )

[0058] σ t =σ e (1+ε e )

[0059] Considering that the material maintains continuity during the deformation process, in order to accurately reflect the plastic deformation characteristics of the material and improve the simulation accuracy, the true stress-strain curve obtained in the plastic deformation stage after the yield point is selected as the material property to be substituted into abaqus.

[0060] The obtained yield strength is shown in Table 2;

[0061] Table 2 Yield strength of various parts of gradient structural steel 2

[0062] Material Name Yield strength(MPa) Material Name Yield strength(MPa) Material 1 921.3 Material 5 512.3 Material 2 694.3 Material 6 511.5 Material 3 532.1 Material 7 501.3 Material 4 502.2

[0063] The specific process of step S2 is as follows:

[0064] The finite element geometric model was established according to the actual tensile specimen size, with the specimen gauge length L = 25 mm, width B = 3.2 mm, and thickness h = 1.8*N (N = 2, 3, 4, 7) mm (when N = 2, it means the composite layer obtained by superimposing and connecting material 1 and material 2 in the model); by constructing a model with the thickness direction of the specimen as the direction of gradient structure change; for materials 1 and 2 with large differences in material properties, in order to reduce the error caused by inaccurate cutting during the processing of the composite layer specimen, the hardness measurement was performed again along the thickness direction of the composite layer specimen and compared with the hardness of the steel block before cutting the specimen to determine the reasonable thickness ratio of the above two material properties in the model. The results are shown in the figure. Figure 5 As shown in (a) and (b) in the figure, the layer thickness of materials 1 and 2 in the model is determined to be 1.2 mm and 2.4 mm respectively. For materials 3 to 7 with similar material parameters, the layer thickness is still maintained at 1.8 mm. The material properties determined in step S1 are substituted into the model, where the elastic modulus E = 222039 MPa and the Poisson's ratio μ = 0.3. The relative deformation and movement between the two adjacent surfaces are constrained by tie connection according to the actual contact area between the components, so that the degrees of freedom of the corresponding two nodes have the same value. The model uses static and general analysis steps. The fixed end is a rigid constraint, and the load application end is a rigid displacement. A transition layer is set at the position where the material properties change, and the material property ΔX of the transition layer is (where X1 and X2 are the basic material properties of the two adjacent interfaces of the transition layer, respectively):

[0065]

[0066] The unit type in the meshing process is hexahedral unit, and the mesh size is 0.4 mm. The transition layer set at the place where the material properties change locally refines the mesh, and the mesh size is 0.1 mm. Figure 6 As shown in (b), the unit grid type is C3D8R;

[0067] The specific process of step S3 is as follows:

[0068] The strain cloud diagram takes the model thickness of 7.2mm as an example, that is, the true stress-strain curves of the first four groups (material 1, material 2, material 3, material 4) are used as the basic material properties; with the end of the elastic stage in the tensile deformation process, the strain shows a trend of transferring from the part with lower yield strength to the part with higher yield strength, until the components are constrained to maintain a similar strain distribution state; at the same time, the high yield strength part with smaller initial strain will bear a larger stress distribution during the deformation process. The strain cloud diagram along the tensile direction is as follows Figure 7 As shown in (b); the load RF1 along the tensile direction and the displacement U1 at the gauge end are extracted from the simulation results of each group, and the simulated engineering stress-strain curve is plotted and compared with the measured engineering stress-strain curve. Fig. 9 As shown, the simulation results presented within the strain range of 5% have great reference value at the macro level.

[0069] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A finite element simulation method for analyzing plastic strain transfer of gradient structural steel, characterized in that: The following steps are involved: S1: Determination of Gradient Structural Steel Material Properties Determine the material properties of each part of the gradient structure according to the change in mechanical properties corresponding to the change in the microstructure of the gradient structure material; S2: Finite Element Geometry Model for Designing Graded Structural Steel Establish a finite element geometric model according to the sizes of different gradient structure samples, assign material properties to different regions of the gradient structure samples, determine the connection mode and boundary conditions, and divide the grid; S3: Analyzing the strain transfer information of gradient structural steel Based on the strain cloud diagram along the tensile direction obtained by simulation post-processing, the strain transfer mode along the direction of gradient structure change during plastic deformation is analyzed, and the simulation results are verified by combining with measured data.

2. A finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 1, characterized in that: In step S1, for the gradient transition of the microstructure presented by the gradient structural steel, the corresponding mechanical properties of different parts are also different. The engineering stress-strain curves of different parts of the gradient structural steel are tested by room temperature tensile test to determine the material properties of each part of the gradient structure.

3. A finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 2, characterized in that: In step S1, in order to accurately reflect the plastic deformation characteristics of the material and improve the simulation accuracy, the engineering stress-strain curve is converted into a true stress-strain curve, and the true stress-strain curve obtained in the plastic deformation stage after the yield point is selected as the basic material property to be substituted into the simulation software abaqus.

4. The finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 2, characterized in that: In step S2, a corresponding finite element simulation model is constructed based on the actual tensile sample size, the material properties of each part obtained in step S1 are sequentially substituted into the corresponding divided areas in the finite element simulation model, and a transition layer is set at the boundary where the material properties change. The relative deformation and movement between two adjacent parts are constrained according to the actual contact area between the parts through tie connection, boundary conditions are assigned, and the mesh size and unit type are determined.

5. The finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 4, characterized in that: The material property ΔX of the transition layer is: Among them, X1 and X2 are the basic material properties of the two adjacent interfaces of the transition layer.

6. The finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 4, characterized in that: The mesh unit type is hexahedral unit. The mesh is locally refined in the transition layer set where the material properties change. The mesh size is 0.1 mm, and the mesh size in the remaining areas is 0.5 mm.

7. The finite element simulation method for analyzing plastic strain transfer of gradient structural steel according to claim 1, characterized in that: In step S3, a strain cloud diagram along the stretching direction is derived after finite element simulation calculation, the continuity of the interface connection is observed, the strain transfer mode along the direction of gradient structure change is analyzed, and the simulation results are verified to calculate their accuracy.

Images