Influence Of Relaxation-Precipitation Controlling Transformation Technique On The Thermo-Stability Of Microstructure In Bainitic Steel

The intermediate transformation products in microalloyed steels obtained by refiningtechnique possess excellent synthetic properties. However, as non-equilibrious microstructures,intermediate transformation products always exhibit a tendency to evolving into equilibriousones during reheated (tempered or reheated in other process). Their excellent properties will belost once they become equilibrious microstructures. For great practical signification of theproblem, it deserves to carry out an investigation on thermo-stability of intermediatetransformation products and the factors determining the property. In the present study,metallographic examination, transmission electron microscopy, hardness measurement alongwith thermal simulation test are employed to investigate the influence of Relaxation-Precipitation Controlling transformation (RPC) technique on the thermo-stability ofintermediate transformation products. Following work is undertaken in this study: Effect of theparameter of RPC technique on the microstructures and mechanical properties of an ultra finelow carbon bainitc steel tempered at different temperature have been investigated, andcompared with those of same composition steels which processed by AC (controlled rolling andair cooling) and RQ (reheating and quenching) technique. The effect of deformation andrelaxation time on microstructure evolution in a simple component steel, which was reheated atdefinite temperatures, were investigated. By comparison with four Fe-Ni alloys, which serve asaustenitic model steels, the evolution mechanism of intermediate transformation products wasdiscussed. It was found that:With the tempering temperature rising, the hardness and strength of all the RPC platesdemonstrate a fluctuation phenomenon, but the amplitude of hardness change and the effect ofhardening are different in samples relaxed for different time. The hardness and strength of ACplate do not obviously changed and those of RQ plate decrease faster as the temperatureincreased. Both of the main microstructures of RPC steel and RQ steel are lath-like bainite andlittle granular bainite before tempering. The microstructure of RPC steel does not changeobviously after tempering, except for the rate of microstructures coarsening are different insamples relaxed for different time. Otherwise, the boundaries of bainitic laths in RQ steeldisappear quickly and finally a polygonal ferrite occurred. These results indicate that the steelswith high strength and toughness manufactured by RPC technique possess high thermo-stability.It is derived from the different states of dislocation and precipitates interaction in steelsintroduced by various techniques.Cooled in water after isothermal relaxation of deformed austenite for different time, a Nb-bearing microalloyed steel always exhibits synthetic microstructures, among which bainiticferrite dominates. Dislocation configurations and distribution of strain induced precipitatesinside bainitic ferrite of samples relaxed for different time are distinct. The strain inducedprecipitates do not occur in no relaxed sample while they distribute outside dislocations insample relaxed for 1000s. The most of strain induced precipitates distribute along dislocationsand pin dislocations in sample relaxed for proper time. When these samples are reheated to andheld at 650℃ or 700℃, the non-equilibrious microstructures tend to evolve into equilibriousones. The sample relaxed for 60s displays the highest thermo-stability, while microstructureevolution is quickest in the sample relaxed for 1000s even though it is softest before reheating.Dislocations inside laths getting rid of pinning of precipitates and their polygonization play theprecursor to the evolution of microstructures during reheating and holding, followed by gradualdisappearance of lath boundaries caused by dislocation climb. Finally, recrystallization happensand polygonal ferrite appears. Pre-strain before reheating accelerates the evolution. By hardnessmeasurement, it is found that softening is not single process occurring during reheating, inwhich hardness fluctuates with time. There are two peaks, which correspond to further growingand coarsening of Nb(C, N) precipitates forming in austenite before intermediate transformationand re-nucleating of the precipitates in intermediate transformation products respectively, inhardness-time curve of each sample having undergone relaxation during reheating at definitetemperature, while single peak occurs in the curve of the sample without relaxed. These resultsindicate that: Thermal stability of intermediate transformation products in Nb-bearing steels isdetermined by interaction state between dislocations and precipitates, rather than by dislocationdensity. The interaction state mainly forms in deformed austenite and maintains afterintermediate transformation.Fe-Ni alloys as austenite model steels can simulate the interaction between the dislocationconfiguration and the precipitates. This interaction is accordant to that of the simple steelsundergone the same deformation and relaxation. These results give further evidence thatthermo-stability of intermediate transformation products in bainitic steels is determined byinteraction state between dislocations and precipitates.