Phase Transformation, Recrystallization Microstructure And Texture During Ultra-rapid Heating Treatment

In this paper, phase transformation, evolution of recrystallization microstureture and texture, as well as formability, for ultra-low carbon steels were exploratory studied. The main focuses were phase transformation and recrystalizaiton microstructure and texture evolution during ultra-rapid annealing process, the main innovative work are as follows:1. The influence of heating rate, initial microstructreu and pre-defromation by cold rolling on the phase transition temperature was first systematic study, and revealing the mechanism of austenite phase transformation in the heating process.The phase transition point in cold rolled steel sheets during heating process is one of key parameters for set the annealing process. However, the influences of parameter during heating process have not been reported systematically although many works have been done on the annealing parameters.In this paper, the heating process was studied systematically by combination thermal simulation, expansion and microstructure observation. The results show that, both of the start transformation point Acl and finished transformation point Ac3of the experimental steel were increased with the heating rate increase, and when the heating rate increases from0.1℃·s-1to300℃·s-1, the rate of Acl and Ac3point increase was not constant increase as the heating rate increases. The highest increase rate can up to12.2when the heating rate between0.1℃·s-1to50℃·s-1, but it will down to0.01when the heating rate above50℃·s-1. As the same heating rate, the Ac1and Ac3point shift to a lower temperature zone when the sample subjected a cold rolling pre-deformation, and the reduction were increase with the pre-defamation increase. Meanwhile, comparing with unreformed samples, the effect of heating rate on the phase transformation point was less than the deformed samples, and under the fast heating rate, the influence of deformation on the phase transformation point was much larger than the slow heating rate conditions. The similar result can be found with the heating rate, that is, the influence of initial microstructure was larger at the fast heating rate than the slow heating rate condition.2. The temperature of recrystallization and evolution of microstructure and texture during ultra-rapid heating were studied, and the influence mechanisms of annealing parameters for recrystallization size and texture evolution were proposed.In this paper, a new annealing technology has been developed in order to conduct fast steel annealing. The microstructure and texture in highly cold deformed to a reduction of94.2%Nb+Ti stabilized interstitial-free (IF) steel and high Nb-IF steel after ultra-rapid annealing (URA) process with heating rates approximately300℃·s-1were characterized by means of OM, TEM, EBSD and XRD. The experimental results indicated that the recrystallization process was significantly accelerated and the finish recrystallization temperature was increased in URA. Moreover, the fully recrystallization can be obtained in as short as～0.41s compared with～4s in conventional annealing (CA) process with heating rates approximately20℃·s-1. In the fully recrystallized condition, the grain size and intensity of{445}<231> fiber in Nb+Ti-IF steel, about11.2μm and15.6, can be observed in an URA cycle, respectively. However, the grain size and intensity of{445}<231> fiber was13.5μm and14.0, respectively, when Nb+Ti-IF steel was subjected to a CA cycle. On the other hand, the URA has no apparently influence on the grain size, within11.0±0.3μm in both URA and CA cycle, of high Nb-IF steel, as less intensity of{223}<472>fiber, about18.0. Simultaneously, more random fiber can be find in an URA cycle than that in CA cycle with higher intensity of{223}<472> texture up to23.9. The grain refining effect in both URA and CA cycle is attributed to the effects of an interaction of nucleation density, annealing time and grain boundary migration rate.3. The influences of non-isothermal annealing on the recrystallization microstructure and texture are studied, and explain the influence mechanism of the parameters of non-isothermal cycle annealing process on the grain size and the recrystallization kinetics.Based on the extensive research works on traditional isothermal annealing process, the whole annealing procedure can be described by the classical recrystallization kinetics. However, there are only a few studies on the non-isothermal annealing although the non-isothermal kinetics reveals a strong effect on the properties of cold rolled steel sheets. In this paper, non-isothermal annealing experiments for two ultra-low carbon steels sheets are carried out. The results show that, the recrystallization process is influenced by the non-isothermal cycle annealing. Grain growth kinetics in cyclic annealing process cannot be revealed by the isothermal recrystallization dynamic formula. Comparing with the grain of cyclic annealing process, non-isothermal annealing can promote the grain growth and improve the distributing of grain size. As the amplitude increase or ramp rate increases, the grain size will increased. However, the grain size will decrease if the amplitude or ramp rate exceeding a certain limit. The study also shows that, in isothermal annealing process, low∑CSL are mainly∑3,∑7,∑9,∑13,∑17and∑19while that in cyclic annealing process are∑3,∑7,∑9and∑13, and the fraction of low low∑CSL is higher than that in isothermal annealing as the almost same grain size. During cyclic annealing processes, the strongest point density and volume fraction of the y-fiber texture are increase with increasing amplitude and ramp rate. When the amplitude increased from30℃to60℃, the density of the strongest point is increase from f(g)=13.2to f(g)=13.7, and the volume fraction of the corresponding point also increased from10.3%to11.7%. With the same amplitude, the density and volume fraction strongest point are increased from f(g)=13.7to f(g)=14.4and increased from13%to13.9%as when the ramp rate increases from1℃·s-1to4℃·s-1. But when the amplitude and ramp rate continue increases, the density and volume fraction of γ-fiber texture are decreased.4. The evolution of recrystallization microstructure and texture of IF steel sheets during annealing are studied. Based on these results, the roles of chemical compositions and annealing process on recrystallization microstructure, texture and formability are explain.The recrystallization annealing experiments show that, the recrystallization start and finish temperature of new Nb-IF steels are higher, nearly30℃, than these in conventional Nb+Ti-IF steel, which is due to more fine two-phase particle pinning grain boundaries and delayed recrystallization occurs for its higher content of Nb and C. Meanwhile, the grain size of Nb-IF steel is uniformity and refinement21%as the same annealing temperature, as well as the low energy grain boundaries has a38.9%increment. The strongest point and inferior point of the new high Nb-IF steel texture are{334}<483> and{554}<225>, respectively, and the peak densities for them are f (g)=13.24and f (g)=11.91, respectively. Refinement grain size, a unique non-uniform gap between the precipitation zone and a textured tissue can be obtained a comprehensive of high tensile strength, elongation, r-value, n-value and good formability.5The variation of microstructure evolution and properties for cold rolled low-carbon BH steels are studied, and the effects of heating rate on recrystallized microstructure and properties of low carbon BH steel are discussed.The bake harding performance of an low carbon steel sheet through control the heating rate to improve the dislocation density introduction and refined grain size is proposed. The research results show that, with the increase of heating rate, grain size is reduced, then, the needed diffusion distance of hot-rolled precipitation particles dissolve is shorten as the decrease of the grain size and tiny grain will increase the volume of C and N solution. In the process of baking, more dislocations are able to pinning and hind the movement of the dislocation, increase the yield strength. However, higher yield strength will reduce the number of dislocation and reduce the BH value when the heating rate increase to300℃·s-1.