Effect Of Austempering On Microstructures And Tensile Properties Of Fe-1.7Mn-1.3Al-0.5C Steels

With the development of automobile industry, the car with light weight, safety and low exhaustion gradually gains the approval of consumers. Consequently, it is foreseeing that the use of high strength steel will become an inevitable trend for the demand of light-weight and low-emission. In this study, the main target is to obtain the better combination of strength and ductility by using heat treatments to adjust the microstructures of Fe-1.7Mn-1.3Al-0.5C steels.The Ac1, Ac3and CCT curve have been determined by the dilatometer. The variation of microstructures and tensile properties has been investigated for the steels after hot-rolling and warm-rolling. Two heat treatment processes air cooling quickly after intercritical annealing (IA) and air cooling after IA and isothermal bainitic transformation (IBT) have been carried out by continuous annealing testing machine and salt-bath furnace respectively. Microstructures and the content of different phases of the steel were investigated by means of optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). On the other hand, mechanical properties of the steel under different heat treatments processes were investigated. The volume fraction and carbon content of retained austenite have been evaluated. The mechanical properties have been tested using tensile machine and Vickers.The experimental results show that Ac1and Ac3temperature of the steel are702℃and863℃respectively. A continuous cooling transformation (CCT) curves have been obtained. After hot rolling, the main phases are ferrite (7-20μm), martensite and pearlite as well as few of cementite. The yield stress and ultimate tensile stress (UTS) are1600MPa and1860MPa, with a total elongation of10%. After740℃warm rolling, the size of ferrite is less than2μm. However, the microhardness decreases from685HV to442HV. The obtained specimen exhibits a yield stress of680MPa and UTS of1140MPa, with a total elongation of17%. With two-stage heat treatment, i.e., intercritical annealing and austempering, the obtained steel mainly consists of ferrite, bainite, retained austenite and martensite. At annealing temperature of800℃, the steel shows an excellent combination of strength and ductility. The yield stress is as high as1110MPa and retains UTS of1250MPa, companied with a superior strain of36%. The product of strength and ductility are45000MPa·%.After annealing at800℃, the volume fraction of bainite decreases with increasing austempering temperature from380℃to450℃with5minutes but that of martensite goes along an inverse way. Meanwhile, the volume fraction of retained austenite increases at first from18%to32%and then decreases to23%with further increasing temperature. The yield stress decreases from660MPa to480MPa while the UTS increases from1200MPa to1470MPa. The total elongation varies from32%to36%and then17%. The above results can be related to that the content of bainite decreases with increasing temperature. During deformation, much more retained austenite transform into martensite, resulting in enhanced elongation and UTS. However, with increasing the austempering temperature to450℃, the increased martensite in the sample will retards the plastic deformation during tension, resulting in a low strain.