| Hot formed parts with, high strength, which can reduce weight of vehicles, energy consumption and exhaust emission pollution to the environment by the premise of guaranteeing the passenger's safety, are widely used in automobile industry. Traditional hot forming quenching process is mainly divided into three stages:austenitizing, forming and quenching and trimming. The mechanical properties of the parts (their ultimate tensile strength are up to 1500 MPa) obtained by the process are uniform. However, there are still some problems:low crash performance, hydrogen-induced delayed cracking and bad connection properties (the total elongation and bending angle by three point bending test is only 5%?7% and 45° respectively), and they are hard for trimming because of their high strength.There is a contradiction between service performance and cutting performance. When the hot formed parts are used in cars, high strength (which can reduce weight) and good elongation (good crash energy absorption ability) are needed. However, low strength is wanted for trimming by tools with high production efficiency and low cost.So, to improve the ductility of hot formed parts under the premise of maintaining high ultimate tensile strength, there are two main ideas:flexible distribution of strength in the parts (Tailored tempering process or Hot forming process for tailored properties) and high product of strength and elongation in parts (hot forming quenching and partitioning process). And to improve the cutting production efficiency by the premise of guaranteeing the service performance, mixed cutting process (including cold cutting in soften zone, hot cutting and laser cutting in high strength zone) is proposed according to the strength distribution of hot formed parts.In hot forming process for tailored properties, according to the three stages (heating, forming and quenching, cutting) of traditional hot forming quenching process, three kinds of method are proposed:tailored heating, tailored cooling and tailored annealing. And the corresponding researches on the microstructure evolutions in these methods are also made. Regardless of what the phase transformation mechanism and process, microstructure of soften zone is predominantly ferrite, supplemented by the hybrid organization (pearlite or bainite and martensite).In the hot forming quenching-static carbon partitioning process, because the contact state (contact order and contact pressure) between blank and tools is different at different areas of the part, the cooling path and the quenching temperature of the part are non-uniform. So, how to make a uniform quenching temperature of the part is the key for this new process or to increase the process window by the material design.In the hot forming quenching-dynamic carbon partitioning process, quenching temperature, which is equal to partitioning temperature, is not constant. And the new process is not sensitive to quenching temperature according to the mechanical properties with different conditions. However the mechanical properties of the new part are lower than that produced by "hot forming quenching and static matching carbon process"; if the mechanical properties are needed to improve, developing new materials,where more retained austenite could be produced in the cooling process by reducing activation energy of carbon across the martensite/austenite interface, is the mian method.In the cutting process of hot formed parts, to improve the cutting efficiency, according to the mechanical properties distribution of the parts and different cutting zone where some position can not be cut by tools because of complex 3D structures, a mixed cutting process (including cold cutting in soften zone, laser cutting in high strength zone) is proposed and studied respectively. according to the strength distribution of hot formed parts, and respective studies are made. And the results can be used as references for cutting process making. |
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