| Energy saving, emission reduction, safety and comfort are the main aims of automotive industry. In order to satisfy the requirement of crash safety and the reduction of car weight, the automotive industry has been putting emphasis upon the application of new materials besides optimizing the frameworks of cars. By using advanced high-strength steels (AHSS) and ultra high-strength steels (UHSS), the car's safety can be improved, and its weight will be reduced remarkably. Whereas, with tensile strength enhancement of AHSS and UHSS, the formability of steel reduces remarkably, and some defects (such as cracking, corrugation, spring-back) easily appear in sheet cold forming.In order to improve the forming performance of UHSS sheets, the hot stamping for the quenchable boron steel sheet was presented and developed in recent years. At the beginning of the process, the blank is put into a furnace, heated up to an austenitization temperature, austenitized for about five minutes to get a homogeneous austenitic microstructure, then transferred quickly to a cooled die for forming and quenching simultaneously, and finally receives a full martensitic structure with an increase in hardness and strength. During the process, the material undergoes three physical changes in microstructure, ferrite+pearlite in blank, austenite in high temperature, and martensite after quenching. By comparison with cold forming proess, the hot stamping process has advantage in the forming of AHSS and UHSS, such as less deformation resistance, good ductility for the blank in high temperature, good formability in complex shapes, good dimensions, little spring-back and high strength.Hot stamping of quenchenable boron steels is a new and complex forming technology combining metal hot forming and quenching process, the key technology is to control the hot plastic forming and quenching with pressure, essentially, the thermal and thermo-mechanical properties like the thermal physical parameters, heat transfer coefficient, the flow curves etc. and the quenching properties of the material are the main factors. So the study of the relative parameters is important for hot stamping technology, especially for a reliable numerical process design by FEM. At present, the research of hot stamping technology is still in primary stage, the thermal property data of materials are needed to be perfect. Thereby it is necessary and important to study the thermal property parameters of boron steel. Then1.6mm sheet of UHSS B1500HS, which is commonly used and quenchenable, is used for research in this paper. Time Temperature Transformation (TTT) curve and thermo-physical parameters of boron steel B1500HS are obtained using the expansion principle and relative models of thermo-physical parameters. TTT curve could obviously describe the phase transformation history of undercooled austenite at isothermal temperatures. The time-temperature characteristics are useful to the practical production and FEM of hot stamping. In order to gain TTT curves, the phase-transformation temperatures of boron steel B1500HS are measured by DIL805A dilatometer. According to the expansion curves of phase-transformation at different isothermal temperatures, the start and finish temperatures of phase-transformation are calculated by using the expansion method and the tangential method. The types of metallographic phases after phase-transformation are judged by the optical microscope and micro hardness, TTT curves are drawn in terms of the results of testing and calculating. For the next, the models of expansion coefficient, density, specific heat, conductivity and modulus of elasticity are built, and the relative specimens of austenitic, ferriticpearlitic, bainitic and martensitic structures are made and measured. As a result of the regression analysis to the relative testing data, the functions based on temperature are respectively obtained for the relative thermo-physical parameters of each phase specimen.A method to research the relative surface heat transfer coefficient (SHTC) in hot stamping process of B1500HS is presented. Due to the characteristics of hot stamping process, the test device to get the temperature data of specimens is designed, the temperature recording system based on the USB port is used, and the inverse heat conduction calculation software developed by research team is improved according to the TTT curves and thermo-physical parameters of B1500HS measured, and used for the relevant SHTC calculation according to the recorded temperature data. The obtained results show, the SHTC between cooling water and cooled die is nonlinear, and it decreases with the die temperature increases, the contact surface temperatures of the specimen and cooled die have little effect but the contact pressure has much effect on the SHTC between the specimen and cooled die. With pressures of1MPa,10MPa,20MPa and40MPa on the cooling system, the result shows that SHTC is approximately linear with relevant interfacial pressure. For advantageous application in FEM, a simplified mathematical model is built using the linear regression method.Flow behaviors of B1500HS are investigated and relative thermodynamics constitutive equations are built based on phase structures, and a new model is presented to describe respectively the flow behaviors of the ferriticpearlitic, bainitic and martensitic structures at lower and middle temperatures. Constitutive equations of material describe the relationships among strain, strain rate and deformation temperature, which are important for building a correct FEM model to improve strain-stress calculation accuracy. In practical manufacture, the phases of austenite, ferrite and pearlite, bainite, martensite of the boron steel could appear due to different cooling rate during hot stamping process. Depending on the TTT curves of B1500HS, the relative phase specimens are made and isothermal tests are performed with strain rate0.01s-1,0.1s-1,1.0s-1and10s-1by Gleeble1500D thermo-mechanical simulator, and the stress-strain curves at the relative deformation temperatures are gained. The modified Arrhenius model, which is a hyperbolic sine function including the deformation activation energy and deformation temperature, is used to describe the hot deformation activation character in austenitic micro structure in the higher temperature range, and a new model is built to describe the ferriticpearlitic, bainitic and martensitic structures in the lower and middle temperature ranges. Simultaneously, the constitutive relationships are confirmed by the comparison of the computational data and experimental data being consistent, the results show the constitutive equations are reasonable.The quenching property of boron steel B1500HS is researched by combining the effects of quenchants, austenitizing temperature, holding time, strain, strain rate or cooling rate on the Ms and Mf microstructure, hardness, tensile strength behaviours of boron steel, and the response surface methodology is used to optimize the key process parameters design. In the experiment, the quenchants are water in room temperature, room air,45steel die and CuCrZr die both with water cooled in channels, The results show that B1500HS has a good quenching performance, the tensile strength and hardness of the samples quenched in a water-cooled steel die are similar to those of the samples queneched with water involved in. The research about the effect of austenitizing temperature and holding time on the mechanical properties of boron steel B1500HS is performed using the water-cooled steel die. The experimental results show that, while austenitizing temperatures is in the range of850-910℃and holding time is5min, the quenched specimens have better lath martensitic structures and quenching properties of higher tensile strength, hardness and ductility. The research is performed about the effects of deformation history (strain, strain rate, cooling rate) on the microstructures, hardness and CCT curves of the austenized specimens. The results show that Ms is insensitive to the strain rate, and sensitive to the true strain, Ms decreases with the strain increasing; the hardness of quenched specimen increases with the cooling rate (above the critical) increasing; at the same cooling rate, when strain increases the hardness decreases due to soft microstuctures transformed, thereby CCT curves may be shifted to the left side with the strain increasing..In order to research the effect of austenitizing temperature and holding time on the hardness, tensile strength and ductility of boron steel B1500HS, the experimental project of two factors and five levels is designed using the austenitizing temperature and holding time as the design factors by design Expert7.0. The experiments are performed according to the design result. The cubic response surface models of hardness, tensile strength and ductility are obtained using the regression analysis prediction method and the results of experiment. The austenitizing temperature and holding time are optimized according to the cubic response surface models, and the optimum quenching parameters are gained.The quenching process FEM simulation system developed by subject team is improved according to TTT curves, the thermal-physical parameters, surface heat transfer coefficient etc. measured in this paper, then embedded in Abaqus software to implement the coupling calculation of temperature-strain/stress-phase transformation during hot stamping process of B1500HS. The hot stamping of typical part is performed to research the effect of blank temperature on the property and microstructure of the final part by the both ways of experiment and FEM simulation, and the die is designed without a blank holder. The experimental and computational results show that austenizing temperature has an obviously effect on the tensile strength and microstructure of quenched part, lath martensites are getting larger with blank austenizing temperature increasing. The microstructures in the flange, side and bottom regions of the quenched part are almost the same for one part. The austenizing temperature has a little effect on the hardness of quenched part. The computational results are almost the same with the relevant experimental results, which shows the TTT curves, thermal-physical parameters, surface heat transfer coefficient etc. obtained in this paper can make the FEM model correct and reliable, the obtained parameters in this paper are useful in the hot stamping process of FEM simulation or practical manufacture for B1500HS sheet.
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