The Research On The Microstructure And Properties Of High Strength Steel Used In Automobile Propeller Shaft

In recent years, the sales volume of the commercial vehicles in China is decreasing andthe competition in the automobile market is becoming much more furious. Propeller shaftassembly is one of the important part in commercial vehicles, and the shaft tube takes up agreat part of the quality of the propeller shaft assy. So the light weight of shaft tubes and theimprovement of the properties of the tubes materials are of great significance. Currently, thesteel used in automobile shaft tube does not have the potential of reducing welded tubesunder the premise of satisfying the bearing strength.In order to achieve the goal of the light weight of propeller shaft and the improvementof the properties of the propeller shaft assembly. Low carbon, microalloying andTMCP(Thermo Mechanical Control Process) are used in this study to produce two kinds ofhigh strength steel used for automobile shaft tube. Tissue of the high strength steel wasanalyzed through simulation and actual measurement the CCT,TTT curve; The type, shapeand distribution of precipitation in steels were studied through Laser confocal microscope,scanning electron microscope (sem), transmission electron microscopy (tem); mechanicalproperties were tested through the tensile test and microhardness test to study the strengthenmechanism and analyze the relationship between different microalloying element contentand yield strength, hardness and toughness, the fracture mechanism was analyzed byscanning electron microscope to observe the tensile fracture morphology; the microstructureof weld heat affected zone and weld fracture mechanism were analyzed by observing theappearance of high frequency weld using laser confocal microscope; hardness distributionfrom the center of weld to the base metal was studied by weld tensile test and microhardnessof weld test to test the mechanical properties of weld. Findings are as follows: microstructureof the two kinds of steel are composed of fine ferrite, Bainite transformation zone of CCTcurve is widen due to the microalloying elements; yield strength levels of two kinds of steelhave reached600MPa grade（C:0.062wt.%,Ti:0.07wt.%,Nb:0.017wt.%）and700MPa grade（C:0.043wt.%,Ti0.08wt.%,Nb:0.057wt.%）respectively, steel which containing muchniobium have much higher strength grade.Niobium element in the steel of600MPa gradestrength is mainly existing in the form of solid solution, the second phase TiC is uniformdistribution within the ferrite grain.700MPa grade steel has more kinds of second phase, which TiC particles and complex titanium niobium carbide distribution in ferrite grainboundary, small amount of large size of precipitated phase TiN exists in steel; throughquantitative analysis of fine-grain strengthening and contrast of the mechanical properties ofthe two kinds of steel, the two main strengthening mechanism of high strength steel arefine-grain strengthening and precipitation strengthening. Because the existence of granularbainite in heat affected zone, the yield strength of600MPa high strength steel welding linehas no obvious improvement compared with that of the former steel used on automobile tube.The heat affected zone of700MPa high strength steel welding line has many granular bainiteand pearlite particles between ferrite grains, so the yield strength of this kind of steel isrelatively high.