| In recent years,environmental pollution,energy shortage and other issues are increasingly prominent.The concept of automobile lightweight promotes the development of advanced high strength steel into a new stage.As a typical representative of advanced high strength steel,TRIP steel has high strength and ductility.In this study,the effects of annealing temperature and rolling process on the microstructure and tensile properties of TRIP steel containing Ce were investigated by using intercritical annealing and bainite isothermal heat treatment.Microstructure,grain size,phase content and element diffusion of Ce-TRIP steel at different conditions were characterized by OM,SEM,TEM,VSM,XRD and EPMA.The mechanical properties of Ce-TRIP steel were measured by tensile test.The main conclusions are as follows:Bainite isothermal treatment(400? for 300 s)of cold rolled 0.05Ce-TRIP steel under different intercritical annealing temperatures were carried out.In the range of 750??880?,with the increase of annealing temperature,tensile strength increased from 662 MPa to 740 MPa,yield strength exhibited slight variation and elongation first increased from 33.6%to 48.1%and then decreased to 43.2%.Therefore,the optimal intercritical annealing temperature was 820?.Compared with TRIP steel without Ce,tensile strength and yield strength of 0.05Ce-TRIP steel after heat treatment at 820? were increased by 100 MPa and 70 MPa respectively,elongation kept constant,and the product of strength and elogation was increased significantly(5.5 GPa·%).Compared with TRIP steel without Ce,the average grain size of ferrite in 0.05Ce-TRIP steel after hot rolling was refined from 10 ?m to 8 ?m,which improved yield strength.With the increase of carbon content in retained austenite,the stability of retained austenite was improved,which enhanced the TRIP effect and improved tensile strength.The microstructure of cold rolled 0.05Ce-TRIP steel after bainite isothermal treatment(400? for 300 s)under different intercritical annealing temperatures was as follows:with the increase of intercritical annealing temperature,the ferrite content decreased gradually,the contents of retained austenite and bainite increased and the carbon content of retained austenite increased first and then decreased.At 820?,the carbon content of retained austenite reached the maximum value of 1.85%.The decreasing ferrite was beneficial to increase yield strength of 0.05Ce-TRIP steel,but reduced elongation.The increasing bainite led to the higher tensile strength of 0.05Ce-TRIP steel at higher annealing temperature.The good combination of content and stability of retained austenite together with the multiphase structure led to the best tensile properties(tensile strength of 736 MPa,elongation of 48.1%)in 0.05Ce-TRIP steel after heat treatment at 820?.Bainite isothermal treatments(400? for 300 s)and intercritical annealing(750??880?)of warm rolled(It was isothermal at 750? for 30 min,heat preservation between passes for 5 min,and other conditions were the same as that of cold rolling)0.05Ce-TRIP steel were carried out.In the range of 750??880?,with increasing annealing temperature,tensile strength of 0.05Ce-TRIP steel first increased from 759 MPa to 817 MPa and then decreased to 807 MPa,yield strength decreased from 564 MPa to 465 MPa and elongation first increased from 41.5%to 44.6%and then decreased to 40.4%.The optimal annealing temperature was 820?,compared with the cold rolled 0.05Ce-TRIP steel after heat treatment at 820?,tensile strength,yield strength,elongation,product of tensile strength and elogation of the warm rolled 0.05Ce-TRIP steel after heat treatment at 820? were increased by 80 MPa,30 MPa,3.5%and 1 GPa·%,respectively.The enrichment of Mn in austenite improved the stability of retained austenite,enhanced TRIP effect and improved tensile strength.The precipitation of Fe3C increased yield strength.The microstructure of warm rolled 0.05Ce-TRIP steel after intercritical annealing(750??880?)and bainite isothermal treatment(400? for 300 s)was as follows:with the increase of annealing temperature,the variation of retained austenite,carbon in retained austenite,ferrite and bainite was the same as that of cold rolled 0.05Ce-TRIP steel.At all annealing temperatures,compared with cold rolled 0.05Ce-TRIP steel,the carbon content of retained austenite in warm rolled 0.05Ce-TRIP steel decreased,and retained austenite and bainite increased.The increase of ferrite grain size resulted in the decreasing yield strength of warm rolled 0.05Ce-TRIP steel at higher annealing temperature.The best tensile properties(tensile strength of 817 MPa,elongation of 44.6%)were attributed to the improved TRIP effect,multiphase structure and fine Fe3C precipitation strengthening. |
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