In-situ Study Of Microstructure Evolution During Recrystallization Annealing Of IF Steel

In this paper,the cold-rolled high strength IF steel of a steel mill production line was taken as the research object.With the high-temperature laser scanning confocal microscope(HT-LSCM)used to simulate continuous annealing of experimental steels,the in-situ study of microstructural evolution during recrystallization was carried out.Combined with the metallographic microscope for metallographic observation,the electron backscatter diffraction(EBSD)technology for recrystallized grain measurement and the Vickers hardness tester for microhardness measurement,the application of in-situ observation technology in the study of the evolution of recrystallized microstructure was explored,and the effects of different annealing processes on recrystallized microstructure and properties of IF steel were investigated.The conclusions are as follows:(1)The appearance of bright color interface and new grains under LSCM observation indicates that austenite transformation from ferrite occurred at this point,while The appearance of the dark interface indicates that recrystallization occurred at this point.The cold-rolled sample of the experimental steel observed by LSCM had an austenitizing start temperature of 855?,a complete austenitizing temperature of 945?,and a recrystallization starting temperature of 705? under continuous annealing.However,due to the delay of hot corrosion,the LSCM observations are slightly higher than the actual ones.(2)The growth of recrystallized grains can be observed in-situ by LSCM.The nucleation site location is at the intersection of grain boundaries,and the nucleation mechanism is continuous nucleation.The grain boundary migration velocity slows down with the extension of annealing time.With the grain boundary being the true grain,the recrystallized microstructure observed by LSCM has a good correlation with the actual microstructure.However,some of the cold-rolled microstructure are embossed layers,and recrystallization may have occurred inside.(3)In the in-situ observation of LSCM,as the holding time increase,the recrystallization progress shows that the area of white cold-rolled microstructure become smaller and transform into the darker recrystallized region with the transition speed getting slower and slower,and then the LSCM images are basically unchanged for the sample has completely recrystallized at this time.As the annealing temperature increase,the grain boundaries in the dark region become more and more obvious for the grain size becoming larger.Under the metallographic observation,as the holding time increase,the grain equiaxedness of the recrystallized microstructure increases;the grain size of the samples is basically larger at first,then decreases first and then increases(but less than the initial stage),and finally remain unchanged due to complete recrystallization.As the annealing temperature increase,the equiaxedness of the recrystallized microstructure increases,which means the recrystallization process speeds up;the equiaxed grain size increases significantly,and the variation range is much larger than the effect of the holding time.The annealing temperature is the main factor affecting the recrystallized microstructure.(4)The LSCM observation and metallographic analysis show that the experimental steel was completely recrystallized under the conditions of continuous annealing at 800?for holding 180 s,820? for holding 120 s,and 840? for holding 60s;after the completion of recrystallization,the holding time has no effect on the recrystallized microstructure.It indicates that HT-LSCM is an advantageous tool for studying recrystallized grain size and holding time.The specific performance is as follows: the grain boundary in LSCM images will be unclear as the grain size becomes smaller,and the images are almost unchanged,which indicating that the sample is completely recrystallized.(5)When the experimental steel is under continuous annealing conditions,with the increase of holding time,the plastic strain ratio r value of the sample increases obviously;the microhardness shows a downward trend,and the higher the annealing temperature,the more obvious the downward trend.With the increase of annealing temperature,the plastic strain ratio of the sample which is just completely recrystallized increases slightly;the microhardness also shows a downward trend,and the decreasing range is basically the same under different holding time conditions.(6)It is feasible to formulate continuous annealing process parameters by HT-LSCM experiments,and it has advantages over traditional methods.According to the complete recrystallization time observed by HT-LSCM experiments and the recrystallization grain size preliminary observed by HT-LSCM experiments and then obtained by metallographic observation,it is found that that the grain size value of the sample annealing at 820? for holding 120 s is more consistent with that of the annealing roll sample completed by the factory's continuous annealing line;and other performance parameters(such as recrystallization texture,microhardness)of the sample annealing at 820? for holding 120 sare in good agreement with that of the annealing roll sample,so the optimal annealing process of the experimental steel was formulated.