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The continue and simulation-batch annealing experiments of fine-grain high-strength IF steels were carried out and annealing processes on microstructure and mechanical properties were studied by optical microscopy, transmission electron microscopy with EDX. The precipitation behavior was studied by observing morphology and analyzing composition for second-phases. The main conclusions are as follow:1.Effecet of continue annealing process on microstructure and mechanical properties of fine-grain and high-strength IF steel1) The average grain size of the test steel, whose Nb/c is 1.8, is about 15μm .It,s found that, the grain have completed recrystallization when annealing time is 2min With an increase of the annealing heating time, ferrite grain coarsen,the size of second phase particle increased,strenth decreased,the value of r increased.2) The average grain size of the test steel, whose Nb/c is 2.9 ,is about 10μm.With an increase of the heating ratio, ferrite grain refined,the size of second phase particle ,strength increased,the value of r increased.3) The average grain size of the test steel, whose Nb/c is 3.8,is about 8μm.With an increase of the cooling ratio, ferrite grain refined,the size of second phase particle increased ,strength decreased,the value of r decreased.2. Effecet of simulation-batch annealing process on microstructure and mechanical properties of fine-grain and high-strength IF steelThe average grain size of simulation-batch annealing test steel, whose Nb/c is 2.9,is about 8~10μm.With an increase of the cooling ratio, ferrite grain refined.the size of second phase particle increased ,strength decreased,the value of r decreased. It,s found that, the grain have completed recrystallization when annealing time is 10min.With an increase of the annealing heating time, ferrite grain coarsen,the size of second phase particle increased, the volume fraction of second phase particle decreased, strenth decreased, the specific elongation and the value of r increased.3. During the annealing process, a much of NbC, Nb(C、N) were precipitated from fine grain high-strength IF steel, and the precipitations would grow by the ways of small precipitations aggregation and merger of the smalls, and the smallers to the big ones when the annealing time increased. The phenomenones of PFZ near the ferrite grain boundaries and segregation of second-phases in lamellar were observed in the IF steels with different annealing processes. Spacing of segregated layers increased with an increase of annealing times. The low yield strength is due to the existence of PFZ and the segregation of second-phases in the Nb-added IF steel which contribute to excellent formability.4. According to this study, Simulation-batch annealing processes of test steel as fllow: annealing temperature: 850℃,annealing heating rate: 3~4℃/s; soaking time: 10~15min, cooling rate: 4~5℃/s. The mechanical propertie can be obtained by chemical component and processing parameter as mention above:σs: 220~250Mpa,σb:410~500Mpa, elongation: 35~37%, Plastic strain ratio: 1.2~1.8, strain hardening index: 0.19~0.22.
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