Research And Application On Key Process Experiment Of High Strength Steel For Hot Forming

Hot forming is a new manufacture technology for high strength steel. It can not only reduce the auto body weight, but also enhance the crash safety, which makes it recognized and widely used in the international automotive industry. Temperature is one of the key factors in the temperature-strain-phase transformation field coupling relationship, which plays an important role in the whole quenching and forming process. Due to the material softening and instantaneous temperature variation, some comprehensive problems occures at elevated temperatures, including formability defects and high-temperature oxidation,etc. The problems happened bring challenges to accurately simulating the heat transfer behaviour between blank and tool surface, as well as predicting the sheet formability. To obtain key process parameters and temperature boundary conditions in hot forming process, further process experimental researchs were executed and contribute to provide essential scientific instruction for optimizing the hot forming process and accurately predicting product formability.In this paper, hot forming process and materials properties were investigated systematically by experiment and simulation. Temperautre field related key technology studies were involved on the process principle, process, materials of boron steel, the temperature field of blank and tool, gradient hardness hot forming technology. Focus on analyzing rules of the material tensile strength, hardness and microstructure properties factors which related to the temperature in hot forming (just like heating temperature, holding time, cooling rate), a series of fundamental tests were executed, KAHN tear toughness experiments with the L9(34) orthogonal design optimization plan were introduced to analysis effective strength and toughness key process parameters with four indexes (tensile strength, elongation, tear strength and UIE) base on comprehensive evaluation method. As a results, improved process parameters were suitable for quenching and low temperature tempering toughness improvement program.To study the thermo-dynamic performance of hot forming boron steel above the martensitic transformation temperature, the independent self-developed thermal mechanical tensile machine was developed to investigate the flow stress and strain relationshipis of different strain rates and temperatures. Five order polynomial fitting equations for boron steel22MnB5rheological stress were obtained by the modified Norton-Hoff model at elevated temperatures, which can be used into the thermal-mechanical field coupling simulation of actual stamping process. Furthermore, thermal forming limit diagram (TFLD) expeiments were executed to get the3D forming limit surface diagram for22MnB5steel at elevated temperature, which providing fracture limit evaluation for formability prediction and temperature field simulation research. In addition, the temperature field experiments and simulation of V-part hot bending and U-part hot forming process were analyzed by controlling cooling rate. Formability experiment and simulation optimization research were conducted by typical deeping drawing parts through controlling the forming temperature in600-700℃, it indicated that the complex production formability improvement and performance meet the requirements in actual production by adjusting cooling rate so as to ensure start forming temperature at optimizing zone.Based on heat transfering and the oxidation mechanism related to the temperature field, a comprehensive analysis of the heat transfer process and experiments were investigated, The high-temperature oxidation kinetics equations and special interface heat transfer coefficient (IHTC) cylinder platform was estabished for obtaining transient IHTC variation law, which considered the influence of pressure and the oxide layer factors through the IHCP reverse modeling optimization process. Relevant heat transfer laws were introduced to self-developed hot forming virtual prototype platform, named KMAS/HF (King-Mesh Analysis System/Hot Forming), to built the hot forming temperature field numerical simulation predict virtual prototype system, the coupled themo-mechanical numerical simulation studies for temperature field and formability was developed to prove the correctness and effectiveness based on relevant interface heat transfer models and simulation methods.This paper also works on the fundamental simulation prediction study and practical experiment for hardness gradient hot forming metal composite materials. The process parameters influence law was investigated on the district cooling hot forming test platform, including tool temperature, air gap distance, etc. Based on the hot forming temperature field simulation platform, a hot forming material hardness-strength-cooling rate index model was established by dimensional analysis and inverse method for implementing the function extension, which not only the temperature field simulation but also the mechanical property prediction function were invovled. The typical shape of the hot forming through experimental and simulation analysis proved that the relevant model can effectively predict the final hardness properties in the actual hot forming process, as well as contributing to provide the foundation for speical hardness gradient property product development.