Study On Influence Mechanism Of Fast Annealing Process On Microstructures And Properties Of Q&P Steel

Quenching and partitioning(Q&P)steel,a new application of the transformation-induced plasticity(TRIP)effect,possesses good strength and ductility.Subject to the conditions of heating equipment,most investigations on the Q&P steel had been performed with alloy design and annealing parameters adjustment.Due to the development of Transverse-field induction heating technology,fast-heating annealing could be applied in industrial production.The fast-heating annealing process would firstly influence the austenitization process of Q&P steels.Thus,microstructure at following process and even the mechanical properties would be different.By using the simulated annealing equipment:Gleeble and Stain,microstructure observation and testing equipment:SEM,TEM,XRD,and quasi-static,high speed,high temperature tensile testing machine,this article studied the effect of fast-heating on the microstructure evolution and mechanical properties of Q&P steels systematically and comprehensively.The main contents and conclusions of the article were as follows:Firstly,the recrystallization behavior of ferrite at different heating rate during continuous heating process was investigated.It was found that the increasing of heating rate postpones the recrystallization of ferrite and makes an overlap exist between ferrite recrystallization and austenite formation process.When heating with 300?/s,the recrystallization behavior of ferrite was almost restrained.The spheroidization degree of pearlite was in proportion with the recrystallization degree of ferrite.A kinetic equation based on JMAK model was established which has high accuracy in describing the volume change of recrystallized ferrite with different heating rate.Secondly,the austenitization process at different heating rate during continuous heating process was discussed.It was found that the nucleation and growth mechanisms of austenite were tied up with the recrystallization degree of ferrite.When ferrite grains were fully recrystallized before transformation,austenite nucleated at the preferential pinned carbides along the grain boundaries,and grew along this grain boundaries.The austenite grain boundaries became the carbon-rich area.At fast heating,austenite nucleated at reserved defects in ferrite grains and grew toward around fastly.The central zone became the carbon-rich area.Dilatometric study and phase transformation kinetics analysis were adopted to analyze this process.Thirdly,the author explored the microstructure and mechanical properties of Q&P steels at different austenitization process.The results indicated that soaking at an intercritial temperature,the austenitization process was accelerated with fast-heating.The tensile strength of fast-heated Q&P steels was markedly increased.Fracture analysis showed that the banded martensite formed in slow-heated sample will lead to the nucleation of voids at an early stage which reduce the plastic property of the steel.For the materials with no internal stress,fast-heating had little influence on them.Refined and polymorphous retained austenite were observed in fast-heated Q&P steel,which would provide more continuous TRIP effect in deformation process.Then,a comparative study was carried out to investigate the influence of quenching and partitioning process on the microstructure and mechanical properties of fast-heated and conventional-heated Q&P steel.With a calculated quenching temperature,Q&P steels had the highest volume of retained austenite.The existing martensite after first quenching would promote the banite transformation,but it was suppressed on the fast-heated sample due to a higher carbon content of austenite.Statistical analysis showed that at elongation between 15%and 27%,the tensile strength of fast-heated Q&P steels raised 90 MPa at average compared with conventional-heated Q&P steels,which realized a wonderful performance ranging from Q&P 980 to Q&P 1180.Finally,two sets of tensile tests with different temperature and strain rate were conducted.The microstructure and mechanics analysis indicated that with the increasing of tensile temperature(-50??300?),influenced by the TRIP effect,high temperature softening,dynamic strain aging and carbide precipitation,the tensile strength of Q&P steel first decrease and then increase.Retained austenite in fast-heated sample had good low-temperature stability,and its mechanical stability characteristics experienced three parts:-50??100?,100??250?,250??300?.With the increasing of strain rate(0.1 s-1?800 s-1),the strength of Q&P steel increased gradually.The conventional-heated Q&P steels had higher strain rate sensitivity.Retained austenite in fast-heated Q&P steels transformed more fully.Taken the temperature rise during deformation into consideration,the modified Zerrilli-Armstrong(Z-A)constitutive model was more accurate than Johnson-Cook(J-C)model in describing high strain rate tensile results.