Research On High Strength Steel Multi-field Coupled Relationships In Hot Forming

High strength steel hot forming is the latest manufacturing technology that integrates with traditional cold stamping and hot forging technology. It is a new technique for modern automotive manufacture to adapt to energy crises, environmental deterioration and increasing safety requirements.High strength steel hot forming technique is based on the large plastic deformation of steel, with high nonlinear mechanical characteristics of geometry, physics and boundary friction. It has the thermo-mechanical-phase transformation multi-field coupled features. Modern hot forming numerical simulation technique is becoming a significant approach to the design of blanks and forming tools, as well as the avoidance of possible manufacturing defect, and has achieved great successes in automotive industry.In the thesis, on the basis of thermo-mechanical-phase transformation coupled constitutive models and experimental analysis of hot forming, the simulation technology of high-strength steel in hot forming is also studied in-depth by the author and his team. Based on the independently developed CAE software KMAS (King-Mesh Analysis System) for sheet metal forming, the static explicit numerical simulation module of hot forming is developed, which considers thermo-mechanical-phase transformation multi-field coupled, nonlinear and large deformation analysis. The typical high strength steel22MnB5hot forming process has been investigated in KMAS, and numerical simulation results include: temperature distribution of blank and forming tools, equivalent stress、equivalent strain and fraction of martensite of blank in hot forming etc. The main research results include:(1) Temperature field:The curves of key thermal parameters (thermal conductivity and specific heat capacity, etc.) are imported into the KMAS hot forming module; In terms of heat transfer theory、general shell finite element and three-dimensional tetrahedral finite element temperature field theory, a coupled heat transfer model for contact interfaces and changing temperature boundaries between blanks and forming tools is proposed; meanwhile, the phase transformation latent heat is introduced into the analysis of temperature field Based on the proposed model, the temperature distribution fields in both blanks and forming tools can be determined. (2) Stress field:The curves of key thermal parameters (elastic modulus, yield stress, etc) are imported into the KMAS hot forming module; Tensile and quenching experiments are implemented at high temperature, then the flow stress curves of sheet metal are introduced into the thermo-mechanical-phase transformation coupled constitutive models to modify numerical simulation results of the stress field.(3) Phase transformation field:According to the thermo-mechanical-phase transformation multi-field coupled relationship, the beginning temperature of martensite transformation and the fraction of martensite transformation are evaluated by KMAS; Optical microscopy images of blank after quenching is agreed with the transformation simulation result.In summary, the numerical simulation results of temperature distribution in U-shaped shell elements and three-dimensional tetrahedral elements is in good agreement with experimental results, which indicates the validity of the coupled heat transfer model. Therefore, the calculated temperature distribution can be utilized to verify the accuracy of thermal analysis and applied to the design of cooling channels for fast and efficient cooling performance. Furthermore, the equivalent stress distribution in U-shaped steel and deep stamping box steel during hot forming process is simulated by KMAS and DYNAFORM software, respectively. It shows that the simulation results in KMAS are in good agreement with that in DYNAFORM.The martensite volume fraction in the final microstructure of blank verifies the accuracy of the proposed multi-field coupled model. By using the KMAS hot forming module, the temperature field、stress field and phase transformation field are simulated. All the above work improved the KMAS multi-field hot forming simulation module, the simulation results can provide accurate prediction for the final formability and microstructure of the high strength steel.