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

Lightweight of automobile is an important way to realize energy conservation andemission reduction, DP steel、TRIP steel and Martensite steel as the representative of AHSS(Advanced High Strength Steel) have been widely applied. Martensitic steel as the commercialization of advanced high strength steel grades the highest intensity, strength in800-1700MPa, is the major bearing parts of the car. In this thesis, the high strength and toughness automotive martensite steel has been fabricated successfully by optimizing the heat treatment process.The static CCT diagram and CHT diagram have been determined by the dilatometer. The samples are heat-treated by salt bath (molten NaCl bath) then quenched into water. The samples are also heat-treated by high temperature box furnace then quenched into water. Microstructure and properties analysis of test steel have been performed using Optical Microscopy, Field Emission Scanning Electron Microscopy, Vickers hardness tester. Mechanical tests have been performed on smpoth tensile samples using a AG-X100KN tensile machine, in order to measure the mechanical properties of test steel.Through the analysis of CCT diagram, the Ms temperature is determined to be at about697K(424℃), when the cooling rate is more than50k/s, martensitic microstructure is observed, at higher cooling rate of175k/s, only martensitic transformation occurs. With the increase of cooling rate, phase transformation temperature declines. Through the analysis of CHT diagram, it shows that the phase transformation temperature increases rapidly with increasing the heating rate from5to75k/s and then slowly from75to150k/s.Through the analysis of high temperature box furnace-quenching heat treatment, for the process keeping the samples at900℃,930℃and960℃for3min, because of grain growth of test steel, mechanical performances are not good. For the process keeping the sample at960℃for3min, the tensile strength Rm is only1188.05MPa, elongation after fracture A is just4.39%.Through the analysis of salt bath-quenching heat-treatmen experiment, for the process keeping the sample at900℃for30s, the tensile strength Rm is1255.13MPa, elongation after fracture A is5.47%; for the other processes, the tensile strength Rm is about1370MPa, elongation after fracture A is about6%.Through the analysis of "zero time holding" salt bath-quenching heat treatment experiment, the tensile strength Rm exceeds1400MPa, elongation after fracture A is about7%under various heat-treatment processes. For the process keeping the sample at920℃for40s, the tensile strength Rm, is1461.23MPa, elongation after fracture A is6.83%; for the process keeping the sample at940℃for30s, the tensile strength Rm is about1468.61MPa, elongation after fracture A is7.23%.