Study On Rapid Heat Treatment Microstructure And Mechanical Properties Of Martensite High Strength Steel

After stepping into the21th century, the auto industry has been achieving huge developments. While under the pressures of energy consumption and environmental pollution, the auto industry is shifted to the directions of safety, energy saving and low-carbon environment protection. The weight reduction of autos is an important way to solve this problem. To achieve this goal, the rapid and continuous annealing process based on magnetic induction heating technology is gaining more and more reputation. The author hopes to optimize the performance of existing steels with this method, realizing the goals of weight and emission reduction of autos.This thesis uses low carbon martensite high strength steel B800NQ as research object, conducting quenching process after heating the samples in high temperature box furnace and salt bath furnace separately. Analyze the microstructure and second-phase particles with metallographic scope, Fe-SEM, TEM, XRD and so on. Test the mechanical properties of samples with electronic universal material testing machine and high strain rate test machine.The research results turn out as follows:In the experiments of heating in the box furnace(quenching after), with the heating temperature of900℃,930℃,960℃for3min, the martensite groups become obvious coarse and poor in mechanical properties performance specially a number of only1180MPa in tensile strength and4.39%in extensibility with the parameters of960℃×3min. In the experiments of salt bath furnace(quenching after), with the process parameters of916℃×20s,916℃×30s,930℃×15s,930℃×20s, the samples gain good mechanical properties with an average number of tensile strength over1400MPa and extensibility number over6%. Especially with the parameter of930℃×20s, the tensile strength approaches1468MPa and extensibility becomes7.23%.In the experiments of heating in the salt bath furnace, although quenching in different ways, the samples’tensile strength all appears decreasing along with the decrease of cooling rate. But processed by930℃、960℃the samples’ tensile strength first drops and then rises a little. And the overall performance of extensibility is a rise at first and then a drop followed. Along with the decrease of cooling rate, more and more carbonization dissolves from the martensite.Under high strain rate, the samples’ mechanical properties improve in the following parameters:processed in box furnace,960℃×3min; processed in the salt bath furnace,930℃×15s、930℃×20s、930℃×30s. With the decrease of strain rate, the samples’ tensile strength drops while the extensibility nearly remains unchanged. In the experiments of heating in the salt bath furnace, processed in930℃×15s as well as a strain rate of103s-1, the samples’ tensile strength is1868MPa and the extensibility is22%.