Research On Hot Forming Of High Strength Steel

Hot forming of ultra high strength steel is the advanced technology that integrates traditional heat treatment and cold stamping. It represents the best solution to increasing the strength-to-mass ratio of sheet components, and then it can meet the need of higher passive safety and weight reduction. The prospect of hot forming technology has attracted the attention of national industrial policy the academic and engineering community, which mainly focus on the high temperature mechanical properties of materials, numerical simulation and experiments. Hot forming technology is systematically researched on these aspects including hot forming technique, theory and experimental methods of hot forming, multi-field coupled constitutive relationships of hot forming, numerical simulation of hot forming, metal composite material in hot forming and their engineering application. The main researching results of this dissertation are in the following:Hot forming techtique:According to the different forming and quenching process, two methods can be employed in hot forming:multi-step method and one-step method. The key technology of the two methods and their die design are studied. The application methods of these two hot forming processes are given. Based on independently developed mass production line of hot forming, the key technology and equipments are proposed, which include die manufacturing with cooling system, continuous heating furnace technology and integrated manufacturing system composed of the advanced interdisciplinary technology of machining, electronic control, material and chemical engineering.Hot forming technique parameters and their optimization method are investigated, which include heating temperature, temperature preservation time, blank transferring time forming speed and die cooling rate. Feasibility methods including theoretical analysis, numerical simulation and experiment are used to the design of the die and its manufacturing process of vehicle HFS (hot forming steel) components. The fast aid optimization method of hot forming parameters is obtained, which integrated numerical simulation of traditional cold stamping and material properties at high temperatures.Theory and experimental research of hot forming:Microstructure and basic mechanical properties of hot forming are introduced. FLD (forming limiting diagram) experiments are carried out at room temperature.The hardening ability of hot forming material is investigated tensile testing at high temperature, which influences material forming ability. The best temperature range for hot forming is obtained. A new experimental method is provided to investigate the anisotropic property due to rolling process of steel.Thermal-mechanical-transformation coupled relations of boron steel are investigated in the hot forming (HFS, hot stamping) process by tensile and quenching experiments at high temperature. In the experiment a plate specimen of boron steel is austenitized for five minutes at 950℃, then tensile forming and quenching take place simultaneously and the force, displacement, expansion and temperature in the experimental process are measured. Based on the analysis of the above physical quantities'variation and the specimen's microstructure, thermal-mechanical-transformation coupled relations of boron steel are researched and the thermal-mechanical-transformation coupled constitutive models are developed. The multi-phase mixed relationship is introduced to analyze the effective thermo-mechanical parameters and mechanical properties of multi phases during the hot forming. The components of strain and their evolved mechanism for hot forming are investigated. By defining the concepts, the phase-transformation volume stress and phase-transformation plastic stress are expressed to explain the mechanism of thermal-mechanical-transformation coupled relations. Based on the above research, the thermal-mechanical-transformation coupled models are introduced into the constitutive equations of hot forming and the integrated and incremental constitutive equations are respectively developed.Numerical simulation of hot forming:Based on the characteristic of hot forming process, the main difference of numerical simulation between hot forming and cold forming is investigated. The key questions in numerical simulation of hot forming are analyzed.Based on developed thermal-mechanical-transformation coupled constitutive models of hot forming, the nonlinear large-deformation dynamic explicit finite element equations are proposed. By defining the concepts, the contact control parameter is expressed to reflect material contact and friction properties at high temperatures. The phase transformation latent heat is introduced into the analysis of temperature field during the hot forming process. Based on the independently developed commercial CAE software for sheet metal forming, named KMAS (King-Mesh Analysis System), the numerical simulation module of dynamic explicit for hot forming is developed.The rate-dependent constitutive equation of hot forming and the finite strain virtual power equation and continuous equilibrium equation based on updated Lagrange method, the large-deformation, multi-field coupled, static explicit finite element equations of hot forming are established. Based on KMAS software, the numerical simulation module of static explicit for hot forming is developed. Metal composite material of hot forming:The multi-layer metal composite material and its manufacturing technique are discovered. The automotive components and parts formed in this technique show that the hardness in the exterior is low and that in the interior is high. The material properties present gradient distributions in the thickness direction. Then the new type multi-layer metallic composite materials is formed. By comparing the crash force and energy absorption between the metallic composite materials and its every single phase material, it is found that the metallic composite materials have the comprehensive performance of every single phase material. So the new metallic composite material is a good alternative material in application of absorbing energy.A new type of metal composite material can be manufactured by controlling heating temperature and designing the layout of cooling pipes in hot forming process of ultra high strength steel. The experimental results show the mechanical properties of the new metal composite material have the characteristics of continuous distribution along the main direction of energy absorption during crash process. The top-hat thin-wall structure consisting of U-shaped metal composite material is employed to analyze the crashworthiness of the new type of metal composite material. The numerical simulation results indicate the new type of metal composite material has the comprehensive performance of every single phase material. So the metal composite is a good alternative material in application of crash resistance.Engineering application of hot forming:The numerical simulation method of impact resistance is provided:according to the constraint and load conditions of the component in the whole vehicle car, the quasi-static load or dynamic load is employed to implement numerical simulation. Based on this method, optimization design of the front bumper beam is carried out by hot forming technology. The results show not only the bumper beam of hot forming improve the impact resistance, but also its weight is reduced by 40%.The optimization design of HFS metal composite material is investigated. The simulation result of B pillar proves the validity of the developed method, which is employed to investigate the impact resistance of HFS metal composite material. The optimization designs of B pillar and S-shape rail are carried out respectively by metal composite material of hot forming technology. Then the hot forming components with better ability of impact resistance are obtained.The new design method-functional design is provided, which applies the parts by hot forming to vehicle body design. Three models are design by using hot-forming parts to compare the numerical simulation results in the vehicle side impacts. Based on the comparison, the mechanism of the improved impact resistance of the whole vehicle by using hot-forming parts is analyzed. The parts by hot-forming should be the overall skeleton layout and form the ultra high strength car-protecting cab and passenger living space. The single part by hot-forming should not be simply employed to the vehicle body design in order that it thrusts into the vehicle body when vehicle crashes. Finally the functional design methods of hot-forming parts used in the vehicle body design are summarized.