Experimental Study On Fatigue Performance Of Q&P Steel Considering Effect Of Stamping And Spot Welding Process

Automobile lightweight is the main way for energy conservation andemission reduction. It draws much attention today when energy crisis andenvironment problem become more and more serious. As a representativeof the third generation advanced high strength steel (AHSS), the tensilestrength of quenching and partitioning steel (or Q&P steel for short) ishigher than800MPa. Automobile weight can be reduced by using thinnersteel sheet. After a special quenching and partitioning heat treatmentprocess, microstructure of Q&P steel at room temperature is martensite andretained austenite. The existence of retained austenite makes Q&P steelhave good plasticity. Q&P steel is a good choice of auto body partsbecause of its combination of high strength and good plasticity. Fatiguefracture is the main failure mode of body parts. Q&P steel must have goodfatigue performance to meet the requirements of vehicle safety anddurability. Up to now, Q&P steel can only be industrialization produced byBaosteel. The lack of performance information of Q&P steel is adverse toits application. Therefore, this paper focuses on the experimental study onfatigue performance of Q&P steel. The main work and conclusions are asfollows:1) Experimental study on fatigue performance of Q&P steel. Fatiguestrength test of Q&P980steel is conducted and fatigue limit is obtained byup-and-down method. Fatigue life is analyzed by statistical methods andS-N curves of different survival rates are obtained. Fatigue crack growthtests of different stress ratios are conducted and fatigue crack growth ratecurves are obtained. Comparison with the other advanced high strength steels shows that Q&P steel has better fatigue performance.2) Martensite phase transformation under cyclic load. Martensitephase transformation of different cycles under cyclic load of differentstress levels is studied by X-ray diffraction method and transformationmodel is established. The result shows that part of retained austenite inQ&P steel gradually transforms into martensite with the increase of cyclingtimes. Velocity and final amount of transformation decrease with thereduction of stress level. When stress level reduces to a certain value,velocity and final amount of transformation on longer change.3) Fatigue performance study of Q&P steel after pre-strain. Fatiguestrength tests are conducted with Q&P steel after uniaxial tensile and planestrain deformation. Effect of different stress states under the same strainand different pre-strain under the same stress state on fatigue performanceof Q&P steel is studied. Influence of strain induced phase transformationand strain hardening behavior in pre-strain process on fatigue performanceis analyzed.4) Experimental study on resistance spot welding process andtensile-shear fatigue performance of Q&P steel. Resistance spot weldingtest of Q&P steel is conducted using power frequency alternating currentservo gun. Effect laws of welding parameters on welding joint quality arediscussed. Hardness distribution and microstructure of welding joint areanalyzed. Fatigue tests of welding joints of different nugget diameters areconducted with hydraulic servo fatigue test machine. Load-life curves ofwelding joints are obtained. Crack growth and failure mode of weldingspot are analyzed. Comparison with the other advanced high strength steelsshows that different advanced high strength steels have the approximatefatigue performance of welding joint.