Study On Air-Mist Quenching And Partitioning Process Of 2000MPa Ultra-high Strength Steel

Under the demand of automobile lightweight,ultra-high-strength steel(UHSS)can be used as the replacement of traditional steel to reduce the weight of the car body significantly.In this article,the UHSS is heat-treated by the quenching and partitioning(Q&P)process,so that it can obtain the product of strength and elongation,excellent comprehensive mechanical properties,and better impact energy absorption characteristics.Aiming at the 2000MPa grade hot-rolled steel of CR2000HS+AS with Al-Si coating produced by Shougang,this essay proposes the one-step Q&P process for heat treatment to improve its comprehensive mechanical properties,and the orthogonal tests were used to determine the quenching cooling rate,partitioning time,and partitioning temperature.Firstly,for the quenching stage in the Q&P process,this essay compared the quenching effects of air-cooling and air-mist cooling and proves the feasibility and necessity of air-mist quenching.Orthogonal tests were proceeded under the air pressure of 0.033MPa,0.067MPa,0.1MPa,0.2MPa,0.3MPa,under which the water flow rate is 0.05L/min,0.1L/min,0.15L/min,0.2L/min respectively.The cooling rate of the samples during quenching was measured by the thermocouple temperature measurement method under different air-water ratios.The air-water ratio of 0.1MPa air and 0.1L/min water flow was finally chosen as the air-mist quenching medium.The average cooling rate was calculated as 22.46?/s.Under the premise of meeting the requirements of quenching performance,a lower cooling rate can make it the reality to precisely control the quenching temperature.Next,for the partitioning stage in the Q&P process,orthogonal tests were carried out at the partition temperature of 225?,250?,275?,300?,325? and partition time of 10s,30s,90s,270s,900s respectively.The microstructures of the specimens were analysed under different partitioning processes.Results illustrate that the volume fraction of the initial quenched martensite and residual austenite can be affected by the partitioning temperature,which provides the kinetic energy for the diffusion of carbon atoms.The partitioning time determined whether the carbon atom diffusion was complete during the partition process,and the volume fraction of martensite and retained austenite after the partitioning was completed.Being treated by the Q&P process,the ideal micro structures at room temperature were martensite and retained austenite.Finally,the tensile strength and elongation of the specimens were tested after the Q&P process under different partitioning times and temperatures.When the partitioning time and temperature were 250? and 90s respectively,the product of strength and elongation was 17.69GPa·%,the tensile strength of the sample was 1466MPa and elongation is 12.07%,which had met the requirement of collision energy absorption capacity of the UHSS.The product of strength and elongation had increased by 125%,compared to that of 7.86GPa·%in the quenching state.This essay has investigated the impact of the Q&P process on the mechanical properties of 2000MPa ultra-high-strength steel.After the Q&P process,the UHSS can obtain a higher product of strength and elongation and good collision energy absorption capacity,which fills the gap of heat treatment of 2000MPa UHSS by Q&P process.Meanwhile,it provides the theory and the solution for the technology of Q&P treated UHSS in the application of automobile lightweight.